High performance VPO catalyst and process of preparation thereof

ABSTRACT

An activated porous phosphorus/vanadium oxide catalyst adapted for the catalytic oxidation of a hydrocarbon to produce a carboxylic acid anhydride. The catalyst comprises a shaped body having a volume of at least about 0.02 cc, a phosphorus content of between about 1.05 and about 1.20, a B.E.T. surface area of at least about 15 m 2  /g, an average vanadium oxidation state of between about 4.06 and about 4.3, a total pore volume of at least about 0.15 cc/g, a normalized apparent shaped body density of between about 1.0 and about 2.0 g/cc, and a crush strength of at least about 4 pounds. At least about 20% of the pore volume of the catalyst is constituted of pores having a diameter between about 0.1 and about 3.3 μM, and at least about 40% of the pore volume is constituted of pores having a diameter of less than about 0.1 μM. The catalyst is prepared by mixing a particulate phosphorus/vanadium oxide precursor with a pore modification agent to produce a modified catalyst precursor composition, forming the modified precursor composition into a predetermined shape, and removing the pore modification agent substantially at a temperature below 300° C., preferably using air as a stripping gas.

This is a continuation of application Ser. No. 08/306,489, filed Sep.15, 1994 now U.S. Pat. No. 5,641,722.

BACKGROUND OF THE INVENTION

This invention relates to phosphorus/vanadium oxide catalysts useful ina process for the oxidation of hydrocarbons to dicarboxylic acidanhydrides, and more particularly to a high surface area catalyst ofimproved microstructure which provides high yields in such a process.The invention is also directed to a method for the preparation of thecatalyst.

Numerous catalysts containing vanadium, phosphorus and oxygen (sometimesreferred to as mixed oxides of vanadium and phosphorus), substantiallyin the form of vanadyl pyrophosphate, optionally containing a promotercomponent, are disclosed in the prior art as being useful for theconversion of various hydrocarbon feed stocks to maleic anhydride. Ingeneral, such catalysts wherein the valence of the vanadium is less than+5, usually between about +3.8 and about +4.8, are consideredparticularly well suited for the production of maleic anhydride fromhydrocarbons having at least four carbon atoms in a straight chain (orcyclic structure). Typically, such catalysts also contain promoterelements or components which are considered to be present in thecatalyst as oxides. Common organic feed stocks include non-aromatichydrocarbons such as n-butane, 1- and 2-butenes, 1,3-butadiene, ormixtures thereof.

Generally, such catalysts are prepared by contacting vanadium-containingcompounds, phosphorus-containing compounds, and promotercomponent-containing compounds (when a promoter element is desired)under conditions sufficient to reduce pentavalent vanadium to thetetravalent state and form the desired catalyst precursor comprisingvanadyl hydrogen phosphate, optionally containing a promoter component.The catalyst precursor is thereafter recovered, typically in particulateform, and subjected to a variety of further conventional processingtechniques to produce the active catalyst. An essential step in suchfurther processing is calcination. Prior to calcination, the catalyst istypically formed into a shaped body such as tablet or pellet bycompression in a die. A lubricant is ordinarily incorporated in theprecursor composition to facilitate the tableting or pelletizingprocess.

In its final form, the catalyst comprises a mass of porous tablets,pellets, other shaped bodies or granules which are charged in bulk toprovide the catalyst bed of a fixed bed reactor. Typically, the catalystis charged to a tubular reactor comprising the tubes of a shell and tubeheat exchanger. Hydrocarbon and oxygen are fed to the tubes, and a heattransfer fluid, such as molten salt, is circulated through the shell toremove the exothermic heat of the oxidation reaction. The porous natureof the catalyst contributes substantially to the active surface area atwhich the catalytic reaction takes place. However, for the internalsurfaces of the catalyst body (tablets or pellets) to be utilizedeffectively, the feed gases, hydrocarbon and oxygen, must diffusethrough the pores to reach the internal surfaces, and the reactionproducts must diffuse away from those surfaces and out of the catalystbody.

It is known in the art that resistance to internal diffusion in thecatalyst bodies can become a rate limiting factor in the reaction. Thediffusion paths can be shortened (and catalyst body external surfaceincreased) by using relatively small catalyst granules. However, in thiscase better mass transfer is purchased at a sacrifice in pressure dropthrough the fixed bed. Thus, a need has existed in the art for aphosphorus/vanadium oxide catalyst having a microstructure such thatinternal diffusion resistance is minimized and productivity is enhancedat constant granule size and pressure drop.

Zazhigalov, et al, "Effect of the Pore Structure and Granule Shape ofV--P--O Catalyst on the Selectivity of Oxidation of n-Butane," ZhurnalPrikladnoi Kimii, Vol. 61, No. 1, pp. 101-105 (January 1988) reportsthat the activity of V--P--O catalysts in the oxidation of n-butaneincreases with an increase in the total pore volume and macroporevolume. Zazhigalov et al further describe the use of polyethylene oxideas a pore forming additive in the preparation of V--P--O catalyst toproduce granules having a greater proportion of macropores. The porebuilder is apparently incorporated in the catalyst precursorformulation, and later removed from the catalyst by burning it out inthe calcination step. This process produces a catalyst having aproportion of macropores significantly greater than was realized withoutthe pore builder. However, despite the previously recognized advantageof macropores, test reactor experiments show that the addition ofpolyethylene oxide resulted in a decrease in the efficiency of thecatalyst. Zazhigalov et al. explains these results by assuming that,after burn-up of the polymer, a dense film of coke remains on thesurface of the catalyst and deactivates the active centers of thecatalyst. They confirmed that hypothesis by the detection of CO₂ thatwas liberated when the catalyst was heated (873° K.) in an air current.

Mount et al. U.S. Pat. No. 4,092,269 is directed to aphosphorus/vanadium oxygen catalyst prepared by reaction of phosphoricacid, phosphorus acid and vanadium pentoxide in an aqueous medium toproduce a catalyst precursor, which is converted to an active catalystby calcination. The catalyst produced contains predominantly pentavalentvanadium, generally having an average vanadium oxidation state of about+4.6 or more. The B.E.T. surface area of the Mount catalyst is about 8m² /g or less, and the pore volume of the catalyst from pores havingdiameters between about 0.8 microns and about 10 microns is greater than0.02 cc/g. Preferably, the volume constituted of pores having diametersbetween 1 and 5 microns is at least about 0.03 cc/g. However, Mountstates that catalysts having a pore volume from pores having diameterslarger than about 10 microns have virtually no effect on the yield ofmaleic anhydride using such catalysts. Catalysts having Mount's desiredfraction of 0.8 to 10 micron macropores are prepared by adding a poremodification agent to the precursor at any stage prior to calcination.Calcination of the precursor containing the pore modification agent isconducted at a temperature between about 300° and 600° C., during whichsubstantial oxidation of tetravalent to pentavalent vanadium takesplace. A lengthy list of pore modification agents is disclosed,including adipic acid, citric acid, oxalic acid, stearic acid,polyethylene glycol, polyvinyl alcohol, polyacrylic acid, cellulosicmaterials, monosaccharides, polysaccharides, hydrogenated vegetableoils, waxes, and gelatin. Cellulosic materials and hydrogenatedvegetable oils are preferred, and methylcellulose especially preferred.The Mount et al. reference states that the yield of maleic anhydrideusing a phosphorus/vanadium oxide catalyst is significantly improved bycontrolling the pore size distribution of the finished catalyst in theranges discussed above. In the working examples of Mount et al., thepore modification agent is removed by calcining at 380° to 500° C.

Bither U.S. Pat. No. 4,699,985 describes the preparation of a maleicanhydride catalyst in which a precursor catalyst is blended with 3 to 5%by weight of an organic pore modifying agent, and with fumed silica inan amount of 0.05 to 0.20% by weight. Upon firing of the blend, theorganic pore modifying agent and the fumed silica generate a catalystmicrostructure which is said to lead to enhanced production of maleicanhydride. Pore builders disclosed as suitable include organic acids,polymeric materials, cellulosic materials, monosaccharides andpolysaccharides, hydrogenated vegetable oils and waxes. A preferred porebuilder is Sterotex hydrogenated cottonseed oil. The pore modifyingagent also serves as a lubricant in preparing shaped catalyst particles.According to the disclosure, the precursor blend is fired in acontrolled manner to generate and activate the catalyst species.Precursor catalyst pellets are heated in a low flow of air at 375° to400° C. for 1 to 6 hours, and thereafter in a more rapid flow of 1 to1.5% n-butane in air at 450° to 490° C. for an additional 16-24 hours.In a preferred method, the shaped catalyst precursor blend is initiallyfired in a continuous zoned belt furnace in an air atmosphere. Thetemperature varies from ambient at the furnace ends to 390° to 395° C.at the center of the heated zone. Air diffuses through baffles at theends of the furnace to replace combustion products diffusing out throughvertical vents located in the heated zone of the furnace.

Methods have been developed in the art for the preparation of highsurface area catalysts by reaction of vanadium pentoxide and aphosphorus compound in an organic medium. The surface area of thesecatalysts is generally in the range of 15 m² /g or greater as determinedby the method of Brunauer, Emmett and Teller, J. Am. Chem. Soc., 60, 309(1938). Surface area as determined by this method is generally referredto in the art as "B.E.T." surface area. Methods for producing highsurface area catalysts are described, for example, in U.S. Pat. Nos.4,632,916; 4,632,915; 4,567,158; 4,333,853; 4,315,864; 4,328,162;4,251,390; 4,187,235; and 3,864,280.

U.S. Pat. No. 5,137,860 describes a process for the transformation of acatalyst precursor represented by the formula:

    VO(M).sub.m HPO.sub.4 aH.sub.2 Ob(P.sub.2/c O)n(organics)

into an active catalyst represented by the formula:

    (VO).sub.2 (M).sub.m P.sub.2 O.sub.7 b(P.sub.2/c O)

where M is at least one promoter element selected from the groupconsisting of elements from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA,IVB, VA, VB, VIA, VIB, and VIIIA of the periodic table, and mixturesthereof, m is a number from zero (0) to about 0.2, a is a number of atleast about 0.5, b is a number taken to provide a P/V atom ratio of fromabout 1.0 to about 1.3, c is a number representing the oxidation numberof phosphorus and has a value of 5, and n is a number taken to representthe weight % of intercalated organic components in the precursor. Intransforming the precursor to the active catalyst, the precursor isheated in an atmosphere of air, steam, inert gas, and mixtures thereofto a temperature not greater than about 300° C. The catalyst precursoris maintained at such temperature under an atmosphere containingmolecular oxygen, steam, and optionally an inert gas, the atmospherebeing represented by the formula (O)_(x) (H₂ O)_(y) (IG)_(z) where IG isan inert gas and x, y, and z represent mol percentages of the O₂, H₂ O,and IG components, respectively, in the molecularoxygen/steam-containing atmosphere, x having a value greater than zero(0) mol percent, y having a value greater than zero (0) mol % but lessthan 100 mol %, and z having a value representing the balance of themolecular oxygen/steam-containing atmosphere. The temperature isincreased at a programmed rate of from about 2° C./min. to about 12°C./min. to a value effective to eliminate the water of hydration fromthe catalyst precursor. The temperature is then adjusted to a valuegreater than 350° C., but less than 550° C. and the adjusted temperatureis maintained in the molecular oxygen/steam-containing atmosphere for atime effective to provide a vanadium oxidation state of from about +4.0to about +4.5. The adjusted temperature is maintained thereafter in anon-oxidizing, steam-containing atmosphere for a time effective tocomplete the catalyst precursor-to-active catalyst transformation toyield the active catalyst.

Andrews et al. U.S. Pat. No. 5,275,996 describes a pore modifiedcatalyst produced using pore modification agents similar to those ofMount U.S. Pat. No. 4,092,269. The catalyst is activated by the methodof U.S. Pat. No. 5,137,860 and has a phosphorus/vanadium atom ratio of1.05 to 1.15, a B.E.T. surface area of at least about 20 m² /g, anaverage vanadium oxidation state of between about 4.06 and about 4.3, atotal pore volume of at least about 0.15 cc/g, a normalized apparentshaped body density of between about 1.0 and about 2.0 g/cc, and a crushstrength of at least about 4 pounds. At least about 5% of the porevolume of the catalyst is constituted of pores having an averagediameter of at least about 0.8 microns, and at least about 4% of thepore volume is constituted of pores having an average diameter of atleast about 10 microns. The pore modification agent is preferablyvolatilized by stripping with an inert gas, containing at least about 5%by volume water vapor, at a temperature in the range of between about150° C. to about 250° C. After 95% to 98% of the pore builder has beenremoved, Andrews teaches the incorporation of up to 5% by volume,preferably no more than 2% by volume, oxygen in the stripping gas toavoid overstripping and undesirable abstraction of labile oxygens fromthe crystal lattice of the V--P--O precursor. During subsequent exposureto an oxygen-containing gas stream during the initial heat up phase ofthe activation process, the temperature is ramped from 300° to 400° C.at a rate of 2° to 12° C. per min.

SUMMARY OF THE INVENTION

Among the several objects of the present invention, therefore, may benoted the provision of an improved phosphorus/vanadium oxide catalysteffective in the catalytic oxidation of hydrocarbons, more particularlythe catalytic oxidation of C₄ hydrocarbons to maleic anhydride; theprovision of such a catalyst which is permeable to relatively rapidinternal diffusion of reactant and product gases; the provision of sucha catalyst which may be used to produce maleic anhydride at highproductivity but low pressure drop; and the provision of a process forthe preparation of such a catalyst.

Briefly, therefore, the invention is directed to an activated porousphosphorus/vanadium oxide catalyst adapted for the catalytic oxidationof a hydrocarbon to produce a carboxylic acid anhydride. The catalystcomprises a shaped body having a volume of at least 0.02 cc, aphosphorus/ vanadium atom ratio of between about 1.05 to about 1.20, aB.E.T. surface area of at least about 15 m² /g, an average vanadiumoxidation state of between about 4.06 and about 4.3, a total pore volumeof at least about 0.15 cc/g, a normalized apparent shaped body densityof between about 1.0 and about 2.0 g/cc, and a crush strength of atleast about 4 pounds. At least about 20% of the pore volume of thecatalyst is constituted of pores having a diameter between about 0.1microns and about 3.3 microns, and at least about 40% of the pore volumeis constituted of pores having a diameter of less than about 0.1microns.

The invention is further directed to a process for the preparation of aphosphorus/vanadium oxide catalyst. In the process, a modified catalystprecursor composition is prepared comprising a mixture of a particulatephosphorus/ vanadium oxide catalyst composition and a pore modificationagent in proportion sufficient to provide a pore modification agentconcentration of from about 8% to about 16% by weight. The poremodification agent is subject to vaporization, decomposition, and/oroxidation at a temperature below 300° C. without leaving a substantialresidue. The precursor composition is formed into a predetermined shapeunder compression, thereby producing a shaped porous catalyst precursorbody comprising the catalyst precursor composition and containing thepore modification agent. The catalyst precursor body is heated whilepassing a stripping gas thereover for removal of at least about 80% byweight of the pore modification agent from the body at temperatures notgreater than about 300° C.

Other objects and features will be in part apparent and in part pointedout hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot comparing the distribution of pore volume as a functionof pore diameter in two vanadium/phosphorus oxide catalysts prepared byANST activation of a tableted catalyst precursor composition, onecomprising a catalyst prepared from a precursor containing no poremodification agent, the other comprising a catalyst of the inventionprepared from a precursor composition containing 10% by weight stearicacid in the pores thereof;

FIGS. 2 and 3 are schematic plan and sectional (along line 3--3)elevation diagrams, respectively, showing an apparatus useful in theremoval of a pore modification agent from a tableted catalyst precursorcomposition and recovery of the pore modification agent for reuse; and

FIG. 4 is a copy of a temperature recording chart illustrating anexcessive exotherm in the removal of pore builder, which resulted in theformation of a catalyst having less than desired activity;

FIG. 5 is a copy of a temperature recording chart illustrating atolerable temperature excursion in removal of pore modification agent inaccordance with the process of the invention;

FIG. 6 is an overlay of several differential scanning calorimetrycurves, one for a tableted vanadium/phosphorus/oxide catalyst precursorcomposition prepared in accordance with the invention, the others fortabletted precursors prepared without a pore builder or with inadequateremoval of the pore builder prior to differential scanning calorimetry;

FIG. 7 is a plot illustrating a correlation between the tablettingpressure at which liquid is extruded from various pore modificationagents and the melting point exhibited by the pore modification agentsin the absence of compressive force; and

FIGS. 8 to 12 are external photomicrographs of catalyst tablets of theinvention showing the incidence of surface holes communicating with theinterior of the tablet; and

FIG. 13 is a plot of catalyst temperature and oven temperature vs. timeduring the preparation of an activated catalyst in accordance with theprocess of the present invention.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the invention, it has been discovered that enhancedproductivity in the conversion of n-butane or other hydrocarbons tomaleic anhydride is achieved by using a high surface area porousphosphorus/vanadium oxide catalyst which has been prepared using a porebuilder to produce a high proportion of large pores therein. Catalystsof the invention contain primarily tetravalent vanadium, having a anaverage vanadium oxidation state of 4.0 to 4.5, preferably 4.06 to 4.3,and a phosphorus to vanadium atomic ratio of between about 1.05 andabout 1.20. The catalysts comprise tablets, pellets or other shapedbodies having volume per shaped body of at least about 0.02 cc and aB.E.T. surface area of at least about 15 m² /g. Such high surface arearesults from a high concentration of pores having an average porediameter of less than about 0.05 microns. By the further presence of ahigh concentration of macropores having a pore diameter in the range ofabout 0.1 to about 3.3 microns, means are provided for rapid internaldiffusion of product and reactant gases within the catalyst body. Thelarge pores constitute flow arteries for distribution of these gases,thereby providing access of reaction gases to the active surfaces of thecatalyst and egress of product gases from the finer pores of thecatalyst body. This rapid exchange of gases allows maximum effective useof more of the internal surface of the tablet or pellet in the catalyticoxidation of C₄ hydrocarbons to maleic anhydride.

Total pore volume of the catalyst of the invention is at least about0.15 cc/g, preferably at least about 0.18 cc/g. The catalyst possesses auniquely advantageous combination of small pores, which present asubstantial active surface for oxidation of a hydrocarbon to maleicanhydride, and larger pores, which provide access of reactants to, andegress of reaction products from, the smaller pores so that maximumeffective use is made of the small pore surface area. Thus, at leastabout 40% of the pore volume is constituted of pores having a diameterless than about 0.1 μM, while at least about 20% of the pore volume isconstituted of pores having a diameter of between about 0.1 and about3.3 μM. In preferred embodiments of the invention, between about 40% andabout 70% of the pore volume is constituted of pores having a diameterless than about 0.1 μm, between about 25% and about 60%, more preferablybetween about 30% and about 50%, of the pore volume is constituted ofpores having a diameter between about 0.1 and about 3.3 microns, betweenabout 10% and about 40%, more preferably between about 12% and about30%, of the pore volume is constituted of pores having an average porediameter between about 0.2 μM and about 2 μM, and between about 5% andabout 20%, more preferably between about 7% and about 15%, of the porevolume is constituted of pores having an average pore diameter ofbetween about 0.5 μM and about 1.2 μM.

FIG. 1 illustrates the distribution of pore volume as a function of porediameter for the catalyst of the invention, as determined by a standardmercury porosimetry test for pore size distribution. For purposes ofcomparison, FIG. 1 also illustrates the pore size distribution for acatalyst prepared without use of a pore builder, but otherwise in amanner similar to that by which the catalyst of the invention wasprepared. Because of the rapid internal diffusion afforded by the poredistribution within the catalyst of the invention, reactant gases candiffuse to the centers of even relatively large bodies for effectiveutilization of the entire internal surface thereof. This allows thecatalyst to be produced in large tablets or pellets, resulting in lowpressure drop through a catalyst bed without sacrificing productivity tothe inaccessibility of internal surfaces. Tablets or other shaped bodieshaving pressure drop characteristics acceptable for commercial fixed bedreactors typically have a principal dimension of at least about 1/8",more typically 5/32" to 1/2". Generally, therefore, the catalyst of theinvention has a volume per body (as defined by the outer contoursthereof) of at least about 0.02 cc, more commonly at least about 0.03cc, and most preferably at least about 0.05 cc. Further to minimizepressure drop in a catalytic reactor containing a fixed catalyst bedcomprising the shaped bodies, the shaped body preferably comprises anopening therethrough for flow of reactant and product gases when thecatalyst is used in the manufacture of maleic anhydride. Advantageously,the shaped body comprise a cylinder having a bore therethrough, asillustrated, for example, in U.S. Pat. No. 4,283,307.

A substantial volume of macropores is obtained by using a poremodification agent in the preparation of the catalyst tablets orpellets. By employing relatively mild conditions in generating themacropores, the desired pore size distribution is realized withoutadversely affecting the activity at the active internal surfaces of thecatalyst. Such advantageous results are realized by both selection ofthe pore modification agent and control of the conditions under whichthe pore modification agent is removed from the catalyst.

Catalysts of the invention have been demonstrated to afford aproductivity of at least about 4 lbs. maleic anhydride per hour-ft³ ofcatalyst, in most instances at least about 4.8 lbs. maleic anhydride perhour-ft³ of catalyst, with productivities of 6 lbs. maleic anhydride perhour-ft³ of catalyst being readily achievable.

Catalysts prepared by the process of the invention also exhibit crushstrengths satisfactory for use in commercial reactors, for example, forthe production of maleic anhydride by catalytic oxidation of n-butane.Gravity and other compaction forces tend to crush porous catalyst bodiesto a powder form, which results in high pressure drop through thecatalyst bed. Inadequate crush strength is generally associated with lowapparent density of the catalyst bodies. Despite their high total porevolume and large proportion of macropores, the activated catalyst bodiesof the invention have been found to exhibit a substantial normalizedapparent shaped body density, in the range of between about 1.0 andabout 2.0 g/cc, and a crush strength of at least about 4 pounds, moretypically at least about 6 pounds. Normalized apparent shaped bodydensity is the same as measured apparent density where the solid phaseof the catalyst is entirely constituted of phosphorus/vanadium oxidecatalyst. Where the solid phase contains a foreign material such as, forexample, a particulate iron aggregate, the normalized apparent densityis determined by adjusting the measured apparent density for the weightfraction of VPO in the catalyst body. Thus, if:

a_(n) =the normalized apparent body density

a_(m) =the measured apparent body density

x=the weight fraction VPO in the catalyst body

then:

    a.sub.n =a.sub.m x

Where no aggregate is present, the normalized (and measured) apparentbody density is between about 1.25 and about 2.0 g/cc.

In the process of the invention, a modified catalyst precursorcomposition is prepared comprising a mixture of a particulatephosphorus/vanadium oxide catalyst precursor composition and a poremodification agent which is subject to removal from the catalyst aftertableting or pelletizing. The pore modification agent should be subjectto removal under mild conditions from the catalyst precursor bodywithout undergoing an exothermic oxidation that adversely affects theVPO crystal structure or composition. More particularly, the poremodification agent should be removable without exposing the precursorcomposition to an exotherm of such severity as to cause eithersubstantial reduction of the vanadium oxidation state or premature oruncontrollably rapid dehydration of the VPO precursor. To facilitate itsremoval under relatively mild conditions, the pore modification agentshould be subject to vaporization, decomposition or oxidation attemperatures below 300° C. and without leaving a carbon, ash, or otherresidue so great as to create a substantial exotherm during thesubsequent catalyst activation process. In particular, it is essentialthat the mechanism of removal of the pore modification agent besubstantially quantitative without generating an exotherm during theremoval process, whether due to decomposition or reaction with theatmosphere in which removal takes place, which would heat the catalystprecursor composition above 300° C. for more than about 20 minutes.

A preferred type of pore modification agent is thermally stable and hasa substantial vapor pressure at a temperature below 300° C. It isparticularly preferred that the pore modification agent have a vaporpressure of at least about 1 mm Hg at a temperature between about 150°C. and about 250° C., more preferably between about 150° C. and about200° C.

Preferably, the pore builder has a relatively high melting point, e.g.greater than 60° C., so that it does not melt during compression of thecatalyst precursor into a slug, tablet or pellet. It is also preferredthat the pore builder comprise a relatively pure material rather than amixture, so that lower melting components are not expressed as liquidsunder compression during formation of slugs or tablets. In the case offatty acids, for example, it is known that lower melting components offatty acid mixtures can be removed as liquids by pressing. If thisphenomenon occurs during slug or tablet compression, the flow of liquidmay disturb the pore structure and produce an undesirable distributionof pore volume as a function of pore diameter.

Particularly advantageous are pore modification agents which have asignificant vapor pressure at temperatures below their melting points,so that they can be removed by sublimination into a carrier gas.

For example, the pore modification agent may be a fatty acidcorresponding to the formula CH₃ (CH₂)_(x) COOH where x>8 such asstearic acid (x=16), palmitic acid (x=14), lauric acid (x=10), myristicacid (x=12), esters of such acids and amides or other functionalizedforms of such acids, for example, stearamide (CH₃ (CH₂)₁₆ CONH₂).Suitable esters may include methyl esters as well as glycerides such asstearin (glycerol tristearate). Mixtures of fatty acids can be used, butsubstantially pure acids, particularly stearic, generally perform betterthan mixtures. While fatty acids and fatty acid derivatives aregenerally preferred, other compositions which meet the functionalrequirements discussed above are also suitable for use as poremodification agents (pore builders).

Other preferred pore modification agents include polynuclear organiccompounds such as naphthalene. Naphthalene melts at about 80° C. and hasan appreciable vapor pressure at temperatures below 175° C. Moreover,because it lacks both functional groups and polarity, it is not stronglyadsorbed, by either physisorption or chemisorption, to the catalystprecursor composition in the catalyst precursor body. Accordingly,quantitative removal of naphthalene is readily achieved under quite mildremoval conditions.

A modified catalyst precursor composition is prepared by mixing the porebuilder in particulate form with a particulate precursor compositionwhich contains oxides of vanadium and phosphorus. Preferably, themodified precursor composition contains between about 6% and about 16%,preferably between about 8% and about 12%, by weight of pore builder.Preferably, a lubricity agent is also included in the mix, e.g., betweenabout 2% and about 6% by weight graphite. This composition is thenformed under compression into a tablet or other predetermined shape.Mixing is better and tablet integrity enhanced if the mean particlediameter of the pore builder approximates the mean particle diameter ofthe precursor composition, at least within about two orders ofmagnitude. Typically, vanadium/phosphorus oxide precursor particles havea mean diameter in the range of between about 20 to 200 microns, mostoften in the range of between about 50 and 150 microns. It is generallypreferred that the mean particle diameter of the pore builder be betweenabout 10 and about 500 microns, more preferably between about 30 andabout 90 microns, most preferably about 40 to about 50 μM.

Although the particulate mixture of vanadium/phosphorus oxide precursorand pore builder can be formed directly into a precursor body which isheat treated for activation, a preferred method involves a preliminaryslugging and granulation step. Thus, the modified precursor mixture iscompressed to produce a slug of material, for example, in the form of acylinder, the slug is then granulated, and the granulated composition iscompressed in a die to form the tablet, pellet or other shaped precursorcatalyst body. Where a fatty acid pore builder such as stearic acid isused, the slug is preferably compressed to a density of between about1.40 and about 1.50 g/cc. Compressing to significantly higher densitiesmay result in undesired expression of liquid phase from the fatty acidpore builder, which may cause a change in the number and sizedistribution of pore builder particles and in turn have a detrimentaleffect on the porosity of the final catalyst product. The tolerabledegree of compression may be generally correlated with the porebuilder's melting point in the absence of compression. FIG. 7illustrates this relationship for a number of exemplary poremodification agents. Expression of liquid is indicated for combinationsof melt point and tablet compression whose co-ordinates fall to theright of and below the solid line of FIG. 7, while expression of liquidis avoided for combinations of normal melting point and tabletcompression whose co-ordinates fall to the left and above the solidline.

Granulation of the slug may be carried out by the mechanical action ofmill knives operated in conjunction with a retaining screen having holeswhich pass the desired size granule. Preferably the granules are on theorder of 200 μM to 1 mm in size, as produced by passage through a screenhaving 1/16" to 1/8", preferably about 3/32" holes. The tablet or othershaped body formed by compression of these granules comprises astructure of mixed particulate phosphorus/vanadium oxide structure andparticulate pore builder. The shaped precursor body has a minimumprincipal dimension of at least about 1/8", preferably 5/32" to 1/2",and thus a volume per catalyst body of at least 0.02 cc, more preferablyat least about 0.03 cc, more preferably at least about 0.05 cc.

The tablets or other shaped precursor bodies are heated under conditionseffective for removal of the pore builder from the bodies withoutcausing an exotherm sufficient to result in excessive prematuredehydration or reduction of the vanadium/phosphorus oxide precursorcomposition. A stripping gas is caused to flow over the shaped bodies toprovide a driving force for mass transfer and to carry away the porebuilder volatilized by heating. A variety of conditions can be utilizedin removal of the pore builder, depending in significant part on itsnature. Generally, it is preferred that the conditions be controlled sothat the precursor bodies are not heated to a temperature higher thanabout 275° C., more preferably not higher than about 255° C. However,brief excursions to higher temperatures can be tolerated so long asneither excessive dehydration nor significant reduction is suffered. Inmost systems, dehydration is not excessive if the temperature does notrise to more than about 300° C. for more than about 5 to about 20minutes. FIG. 5 illustrates a modest exotherm of the type that can betolerated during removal of pore builder. By contrast, FIG. 4illustrates an excessive exotherm which damages the catalyst structure.

Where the pore builder has a melting point significantly lower than thetemperature at which it is removed, the precursor bodies are preferablyheated to a temperature above the melting point but below the lowesttemperature at which the pore builder is subject to substantial thermaldegradation or oxidation by the catalyst precursor or components of theatmosphere to which the shaped body is exposed during heating.Preferably, the shaped body is heated to a temperature at which thevapor pressure of the pore builder is above 1 mm Hg, but well below 300°C. Most of the pore builders of interest may be removed at or nearatmospheric pressure by heating to a temperature in the range of betweenabout 150° C. and about 250° C. Where the pore builder has a vaporpressure of <1 mm Hg in the range of between about 150° C. to about 200°C., the shaped body is preferably heated, at gradually increasingtemperature, to a terminal temperature in the range of between about200° C. and about 250° C. The stripping gas preferably flows over thecatalyst body at a velocity of at least about 25 cm/sec., morepreferably between about 50 and about 75 cm/sec. Satisfactory removal ofpore builder can be realized over a wide range of pressure, includingatmospheric and above. A pressure slightly below atmospheric ispreferred.

In order to prevent excessive dehydration of the VPO structure, it isimportant not only to avoid excessive exotherms that can result fromoxidation of the pore builder with oxygen from the stripping gas whenthe catalyst precursor is heated too fast, but also to remove as muchpore builder as possible at relatively low temperature, below thetemperature at which oxidation of the pore builder may result fromabstraction of oxygen from the VPO catalyst precursor structure. Thelatter reaction is a particular risk towards the end of the pore builderremoval step, at the relatively high temperatures that may be requiredfor lowering the concentration of residual pore builder to a level atwhich no serious exotherm is experienced in the subsequent catalystactivation stage. Moreover, fairly lengthy exposure to such temperaturesmay be necessary to eliminate residual pore builder to the extentrequired for the activation step, without generating an exotherm in theremoval step by heating too rapidly. Thus, for example, where the shapedprecursor bodies need to be heated to a temperature of 240° C. forremoval of residual pore builder by stripping, they are exposed topotential dehydration and reduction not only during the approach to 240°C. but also during the several hours typically required to cool to atemperature below about 150° C. At such temperatures, if the amount ofresidual pore builder remains too high, the catalyst precursor structureis subject to both dehydration and reduction due to reaction of the porebuilder with oxygen abstracted from the crystal VPO lattice. Moreover,where air or other oxygen containing gas is used as the stripping gas,and a substantial exotherm is experienced, the temperature of the VPOprecursor may rise so high that once the pore builder has burned off,the catalyst structure becomes oxidized by reaction with oxygen in thestripping air, leading to final vanadium oxidation state well above thedesired range. Thus, as discussed in more detail below, the bulk of thepore builder is preferably removed at a temperature below thetemperature at which it may be subject to rapid oxidation.

It is also important to achieve substantially complete removal of thepore builder before heat treatment of the precursor bodies to produceactivated catalyst bodies. When exposed to oxygen during the subsequentheat treatment for activation, residual pore builder may oxidize at anexcessive rate, causing an excessive exotherm in that operation whichmay interfere with the conditions of the heat treatment. Thus, forexample, when using the preferred method of activation as describedbelow, it is important that, in the range of about 300° to about 400°C., the rate of increase in the temperature of the precursor bodies isnot faster than about 2° to about 12° C. per minute. If any significantresidual amount of pore builder is present, an exotherm may developwhich increases the heating rate of the precursor bodies to well above12° C. per minute, even where they are heated in an oven in which therate of increase of the air or other stripping gas temperature is wellwithin the 2° to 12° C./min. range. Accordingly, the removal step shouldbe continued until at least 80%, preferably at least about 85%, of thepore builder has been removed, as indicated, for example, by measuredweight loss during the removal step.

Severe exotherms can also be experienced in the pore builder removalstep, so that the rate of temperature increase must be carefullycontrolled in that step as well. Many pore builder materials are subjectto uncontrolled oxidation if the precursor bodies are heated toorapidly, especially at temperatures in the upper portion of the range inwhich pore builder is removed. Rapid oxidation may result fromabstraction of oxygen from the crystal lattice, but may be much moresevere where air is used as the stripping gas. It has been observed thatthe precursor bodies can reach temperatures as high as 600° C.,essentially destroying the desired crystal structure of the catalyst.Even at temperatures well below 600° C., a substantial exotherm mayresult in excessive dehydration of the crystalline VPO structure. In thepresence of air the oxidation state of the vanadium may be increasedsubstantially above the preferred 4.06 to 4.3 range, while in theabsence of air, the vanadium oxidation state may be reduce below thatrange. FIG. 4 illustrates an excessive exotherm of the type that may begenerated in the course of pore builder removal.

The tendency to uncontrolled oxidation and generation of excessiveexotherms is believed to be a function of the nature of the porebuilder, the temperature and the amount of pore builder remaining in theprecursor bodies at a given temperature. Thus, the heating rate may becritical. It has further been observed that certain pore builders aresubject to catalytic oxidation in the presence of the VPO precursorbeginning at a threshold temperature which varies with the nature of thepore builder. At this temperature, it is understood that oxygens of theVPO crystal lattice become labile and react with the pore builder.Oxygen withdrawn from the lattice by this reaction is replenished byoxygen transferred from the stripping gas. If the amount of remainingpore builder is low when the threshold ("light off") temperature isreached, catalytic oxidation provides a useful and beneficial means ofsubstantially eliminating residual pore builder prior to hightemperature heat treatment for transformation of the precursor to activecatalyst. This phenomenon may provide a particular benefit when air isused as the stripping gas. In such instance, the pore builder oxidationproceeds at a controlled rate, causing neither excessive dehydration,reduction of vanadium, oxidation of vanadium, or adverse alteration ofthe crystal structure. However, if the amount of remaining pore builderis too great, catalytic oxidation thereof may generate an excessiveexotherm. Thus, it is desirable to remove as much of the pore builder aspossible at relatively low temperature.

In a preferred embodiment of the invention, therefore, the precursorbodies are initially heated at a modest rate, for example, at a ratesuch that their temperature increases at between about 1° C. and about3° C. per minute to a hold temperature at or slightly lower, preferablyno more than about 15° C. lower, than the light off temperature forcatalytic oxidation of the pore builder in the presence of theprecursor. Operation at the light off temperature is acceptable becausethe oxidation rate at this temperature is slow enough that the reactionheat can be readily removed by the stripping gas, and both bulk andlocal hot spots are avoided. The precursor bodies are then maintained atthe hold temperature for a period of time during which additionalamounts of pore builder are removed by evaporation. During the initialheatup and the hold period, the stripping gas serves as a heat source,and the pore builder is removed essentially by evaporation orsublimation, while oxidation is substantially avoided. After the holdperiod, heating of the precursor bodies is resumed at a very slow rateto the light off temperature, and beyond the light off temperature tothe terminal temperature of the pore builder removal step. During thelatter phase of the process, it is believed that residual pore builderis removed both by evaporation and catalytic oxidation. In this period,the stripping gas typically serves as a heat sink rather than a heatsource, as is indicated by the fact that the temperature of the catalystbodies ordinarily exceeds the temperature of the stripping gas once thelight off temperature for the pore builder has been reached.

Where stearic acid is used as the pore builder, air is preferably usedas the stripping gas, and the precursor bodies are heated to increasetheir temperature at a rate of about 1° C. to about 3° C. per minutefrom the initial temperature, typically ambient, to a hold temperaturenot greater than about 170° C., preferably about 165° C. At thevelocities discussed above, the air is advantageously recirculated forconservation of energy, a fractional purge stream having a flow volumeof 5% to 20% of the circulating rate being withdrawn for disposal ofpore builder and pore builder oxidation products. The bodies aremaintained at the hold temperature until enough pore builder has beenremoved so that the heat generated by subsequent catalytic oxidationdoes not cause an excessive exotherm. More particularly, the catalystbodies should be maintained at the hold temperature long enough so thatany exotherm in the remainder of the pore builder removal step does notexceed a maximum temperature of about 300° C. If desired, pore builderremoval during the hold period is monitored by weight loss. After thehold period, heating to increase the temperature of the precursor bodiesis resumed at a very gradual rate. The hold period is typically betweenabout 1 to about 10 hours, depending on the nature of the pore builder,the initial pore builder content, and the nature and velocity of thestripping gas. At a volumetric air flow velocity of between about 0.5and about 20, more preferably about 4 to about 10, liter/sec.-kgcatalyst precursor, and a linear velocity of between about 50 and about75 cm/sec, the hold period for stearic acid is preferably between about1 and about 7 hours.

After the hold period, the catalyst bodies are heated at a rateeffective to progressively remove residual pore builder by oxidationand/or vaporization while avoiding the generation of an excessiveexotherm. The rate of removal is a function of the amount of residualpore builder and the temperature. As the temperature increases, theproportion of catalyst sites that are active for oxidation of the porebuilder may be visualized as progressively increasing. At a giventemperature, the rate of removal of pore builder by oxidation and/orvaporization declines as pore builder is consumed, until theconcentration of residual pore builder asymptotically approaches alimiting, essentially irreducible value. Thus, to achieve progressivereduction in pore builder concentration beyond the asymptotic limit at agiven temperature, the temperature must be progressively increased.However, the rate of oxidation may become uncontrollably high if thetemperature is raised too high or too rapidly at a given residual porebuilder concentration. It is critically important that the oxidationrate not reach the point at which local hot spots adversely alter VPOphase structure, or even reach a self-accelerating level at whichreaction heat generation rate exceeds the rate of heat removal in thestripping air flowing over the precursor tablets. As a consequence, itis necessary to increase the temperature very gradually to maintain areasonable rate of removal of residual pore builder while avoiding anexcessive exotherm. It has been found that, as the pore builderconcentration decreases, it becomes feasible to moderately acceleratethe rate of heating until a terminal temperature is reached at which theasymptotic concentration of residual pore builder is low enough so thatan excessive exotherm is avoided in the subsequent catalyst activationstep.

The exact time/temperature schedule may vary with such parameters as theidentity of the pore builder, the stripping air flow velocity, the sizeand shape of the precursor tablets, the surface to volume ratio of thebed of catalyst bodies in the pore builder removal oven, and the totalweight of catalyst precursor bodies relative to the volumetric air flowin the oven. Typically, however, the precursor bodies are heated veryslowly, for example, from about 0.5° C. to about 3° C. per hour,preferably about 1° C. to about 2° C. per hour, to a temperature in therange of about 15° to 40° C. above the hold temperature, and thereafterat a rate increasing to 10° to 40° C. per hour, preferably about 15° toabout 25° C. per hour, at temperatures ranging from around 200° C. tothe terminal temperature (240° to 260° C. in the case of stearic acid).Thereafter, the precursor bodies are cooled to ambient temperature,preferably in about 0.5 to 2 hours. The precise temperature schedule iscontrolled so that the vanadium oxidation state is not reduced to below3.8. For example, a large batch of precursor bodies containing a stearicacid pore builder may be heated on the following schedule:

    ______________________________________                                        ambient to 170° C.                                                                            1 hour                                                 170° C. hold    2 hours                                                170° C. to 185° C.                                                                    15 hours                                                185° C. to 199° C.                                                                     7 hours                                                199° C. to 255° C.                                                                     2 hours                                                cool to ambient        2 hours                                                ______________________________________                                    

A smaller batch may be treated on a somewhat more aggressive schedule,for example

    ______________________________________                                        ambient to 165° C.                                                                            1 hour                                                 165° C. hold    2 hours                                                165° C. to 199° C.                                                                    17 hours                                                199° C. to 255° C.                                                                     2 hours                                                cool to ambient        2 hours                                                ______________________________________                                    

The entire cycle has been found to extend for 15 to 35 hours, optimally23 to 28 hours.

Removal of pore builder using air as the stripping gas, and followingthe temperature schedule outlined above, has been found to avoid theproblem referred to as "overstripping" in Andrews U.S. Pat. No.5,275,996. Whether overstripping results from an excessively reducingatmosphere, from oxygen abstraction resulting from bonding between theVPO substrate and carboxyl groups of a fatty acid pore builder,thermally initiated oxygen abstraction reactions, or both, it has beenfound that this problem is avoided in the process of the presentinvention wherein the heating rates are very strictly controlled and airis used as the stripping gas.

By use of air as a stripping gas under the controlled temperatureconditions of the invention, even non-volatile pore builders such asmethyl cellulose may be effectively used without adverse effect on thefinal properties of the catalyst. According to the methods of the priorart, it was necessary to burn out such non-volatile pore builders in afashion which resulted in an excessive exotherm. More specifically,catalyst bodies containing cellulosic type pore builders were heated torather elevated temperatures in the presence of air, causingdecomposition of the pore builder into volatile fragments which werethen burned in the air, causing the temperature of the catalyst body torise far above 300° C., with consequently excessive and prematuredehydration. However, by stripping with dilute oxygen-containing gasunder conditions below the flammable range, cellulosic pore builders canbe entirely removed at temperatures substantially below 300° C. withoutexcessive dehydration or other adverse effect on the precursor crystallattice.

As indicated, brief temperature excursions as high as 300° C. may betolerated if the energy absorbed by the precursor body is not such as tocause excessive dehydration during the removal step.

Where the pore builder comprises a relatively volatile compound such asnaphthalene, or other polynuclear organic which is free of polarity orfunctional groups that would generate high energy bonds with theprecursor substrate, it may be removed by stripping at temperatures,below about 170° C., at which the oxygen atoms of the precursor crystallattice are essentially non-labile and the vanadium is not subject toreduction in its oxidation state. Where the pore builder is removable atsuch modest temperatures, it may not be necessary to heat to thethreshold temperature at which catalytic oxidation of the pore builderis initiated in the presence of the catalyst precursor. However,provided that the substantial bulk of the pore builder is removed byevaporation below the light off temperature, catalytic oxidation may berelied on to remove residual pore builder.

Regardless of which of the above described methods is used, the porebuilder is removed from the shaped porous body, leaving a porous,gas-permeable structure containing a substantial fraction of macropores,as discussed above. When the pore builder is removed using the extendedheating cycles described hereinabove, any residue of carbon, ash oradsorbed organic species at the internal surfaces of the catalyst isreduced to a practical minimum. Thus, when the precursor is subsequentlytransformed into active catalyst at temperatures in excess of 300° C.,the highest feasible catalyst activity is realized. More particularly,the catalyst surfaces are not exposed to uncontrolled exothermictemperature excursions which may otherwise cause the catalysttemperature to rise more rapidly than about 2° to 12° C. per minuteduring the ramp heating from 300° to 400° C. as described in more detailhereinbelow.

Where a pore builder is burned out of a catalyst tablet by previouslyknown methods, the internal surfaces are exposed to combustiontemperatures, which are substantially in excess of the temperatures atwhich transformation of the catalyst precursor to active catalyst iseffectively conducted. By contrast, in the process of the invention, thebenefit of a substantial distribution of macropores is achieved, whilethe adverse effect of carbon deposits, physisorbed or chemisorbedorganic species, or exposure of the pore surfaces to excessivetemperatures, is avoided. Residual carbon or organic material may not beentirely eliminated, but the residual amount per unit surface area issmall enough that, by careful control of the heating rate during thecatalyst activation step, the residual carbonaceous material is oxidizedat rate which does not cause a substantial exotherm, and does not resultin either reduction of vanadium, untoward dehydration of the VPOprecursor, loss of effective B.E.T. surface area, or adverse effect onthe VPO crystal structure.

A system for removal of a pore builder is illustrated in FIGS. 2 and 3.In the apparatus shown, an oven 1 contains a pair of racks 3 and 5, eachrack holding a plurality of trays 7, 9, and each tray in turn containinga relatively thin layer of shaped porous bodies 11 comprising thecatalyst precursor composition containing the pore builder within theirpores. Stripping air is circulated through oven 1 and an indirect heatexchanger 13 by means of a blower 15. Air exiting blower 15 is dividedbetween a gas duct 17, which communicates with an air inlet 19 on oneside of oven 1, and a gas duct 21, which communicates with an inlet 23on the other side of the oven opposite inlet 19. Rack 3 is located onthe side of the oven adjacent inlet 19, rack 5 is located adjacent inlet23, and a space between the racks constitutes a plenum 25 for collectionof air circulated over the trays for return to the blower via a gas exit27 of the oven and heat exchanger 13.

Each rack 3, 5 is open on the sides thereof facing the walls of the ovenin which the stripping gas inlets 19 and 21 are located, but is coveredwith sheet metal walls 29, 31 on the top bottom, and the sides facingthe other two walls of the oven. Thus, an enclosed "tunnel" is formedhaving one side open for inflow of stripping air and an opposite sideopen for exit of air containing pore builder stripped from the shapedcatalyst bodies. On the inlet ("upwind") side of each rack, the sheetmetal walls are extended beyond the edge of the rack and flaredoutwardly to form a louver 33, 35 to capture as much air as possiblecoming from the oven recirculating blower and direct it into the"tunnel" formed by walls 29, 31, thus providing the most desirable airflow over the catalyst precursor shaped bodies on the trays in the rack.Though not indicated in the drawing, each rack may contain two verticalarrays of trays, one adjacent the upwind and the other adjacent thedownwind openings in the "tunnel."

In accordance with the process of the invention, air or other strippingor purge gas is passed over trays 7, 9 of shaped porous bodies forremoval of vaporized pore builder from the shaped bodies. The strippingair is heated by indirect heat transfer in heat exchanger 13, the heatof vaporization of the pore builder being provided by direct heattransfer from the stripping air. The stripping gas flows over the traysat a velocity in the range noted above. In a preferred embodiment, asillustrated, the gas flows both over and under a relatively thin, e.g.,1" thick layer of precursor bodies. The pressure of the stripping gasand temperature of the shaped bodies falls within the ranges outlinedabove. During the concluding phase of the pore builder removal cycle,oxygen in the stripping gas may participate in controlled catalyticoxidation of residual builder while preventing abstraction of labileoxygens and reduction of vanadium. As noted above, air is preferablyused. To conserve energy the air is recirculated by means of a blower15. A purge stream is discharged through line 37 and valve 39, whilemakeup air is drawn in through line 41. If desired, the purge may beregulated on a programmed basis at a rate proportional to the rate ofvaporization/oxidation of pore builder, but is conveniently set at afixed rate which preserves a substantial driving force for mass transferduring periods in which the rate of pore builder removal is at amaximum. The purge stream is conveyed to a combustion chamber where itis burned at high temperature to produce an innocuous exhaust gascomprising water vapor and carbon dioxide.

After removal of the pore builder, the shaped catalyst body is subjectedto calcination or other heat treatment to convert the catalyst precursorcomposition to active catalyst. Preferably, the transformation to activecatalyst is carried out in the manner described in copending andcoassigned application Ser. No. 07/722,070, Attorneys' Docket No. 24-21(7928), which is expressly incorporated herein by reference. Thisapplication describes catalysts having a composition represented by theformula

    (VO).sub.2 (M).sub.m P.sub.2 O.sub.7 b(P.sub.2/c O)

wherein M is at least one promoter element selected from among elementsof Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB, andVIIIA of the Periodic Table of the Elements, and mixtures thereof, m isa number from zero (0) to about 0.2, b is a number taken to provide aP/V atom ratio from about 1.0 to about 1.3, and c is a numberrepresenting the oxidation number of phosphorus and has a value of 5.The oxidation state of the vanadium is between about 4.0 and about 4.5,preferably between about 4.06 and about 4.30. The activated catalyst hasa B.E.T. surface area of at least about 15 m² /g, preferably at leastabout 20 m² /g. By incorporating a substantial fraction of macropores,the activated catalyst provides avenues for access of reactant gases tothe extensive active surface contained in what is also a highconcentration of fine pores, and egress of product gases from the pores.

Although catalysts, as represented by the above formula, are indicatedas having a phosphorus-to-vanadium (phosphorus/vanadium or P/V) atomratio of from about 1.0 to about 1.3, preferably from about 1.0 to about1.2, most preferably from about 1.05 to about 1.15, the actual P/V atomratio may range from a value as low as about 0.9 up to the stated valueof about 1.3. The total atom ratio of promoter element-to-vanadium(promoter element/vanadium or M/V), when a promoter element is presentas a component of the catalyst, advantageously is in the range fromabout 0.0001 to about 0.2, preferably from about 0.0005 to about 0.1,most preferably from about 0.001 to about 0.05. These catalysts exhibitenhanced catalyst activity and excellent selectivities to and yields ofmaleic anhydride when compared to catalysts transformed from catalystprecursors via conventional procedures. Further enhancement of activityis provided by the use of a pore modification agent to produce highfractions of macropores as described above.

Catalyst precursors suitable for use in the process of the instantinvention are those known in the art and in general are materialscapable of being transformed in accordance with the process of theinvention into active catalysts which are capable of catalyzing thevapor phase partial oxidation of non-aromatic hydrocarbons to maleicanhydride under oxidation conditions. Such catalyst precursors arerepresented by the formula

    VO(M).sub.m HPO.sub.4 aH.sub.2 Ob(P.sub.2/c O)n(organics)

wherein M, m, b, and c are as defined above, a is a number of at leastabout 0.5, and n is a number taken to represent the weight percent ofintercalated or occluded organics component. The catalyst precursor maybe prepared, for example, in an organic reaction medium such as primaryand secondary alcohols--methanol, ethanol, 1-propanol, 2-propanol,2-methyl-1-propanol (isobutyl alcohol), 3-methyl-2-butanol,2,2-dimethyl-1-propanol, 1,2-ethanediol (ethylene gylcol), for example.Intercalated or occluded organic materials (organics), as represented bythe term "n(organics)" in the formula for the catalyst precursors, mayrepresent up to 40% by weight, or higher, typically from about 2% byweight to about 25% by weight, of the catalyst precursor composition,depending upon the conditions (temperature and time) under which thecatalyst precursor is dried. For example, if the catalyst precursor isdried at about 150° C. for about 8 hours, the intercalated organicmaterials typically represent about 25% by weight, while drying at about250° C. for about 4 hours typically results in a catalyst precursorhaving about 2% by weight intercalated organic materials. In general,the preparation of the catalyst precursors in an organic reaction mediumis preferred over preparations carried out in an aqueous medium. Mostpreferred among suitable organic reaction media are the previously notedprimary and secondary alcohols, with isobutyl alcohol being mostpreferred.

Specific, albeit nonlimiting, examples of suitable catalyst precursormaterials are those described in U.S. Pat. Nos. 4,632,916; 4,632,915;4,567,158; 4,333,853; 4,315,864; 4,328,162; 4,251,390; 4,187,235;3,864,280; and European Patent Application No. 98,039--it beingunderstood however, that the same are not to be construed as limitingbut instead are for purposes of illustration and guidance in thepractice of the process of the instant invention. These references areherein incorporated by reference. Among such catalyst precursormaterials, nonlimiting examples of those preferred for use in theprocess of the instant invention are those described in U.S. Pat. Nos.4,632,915 and 4,567,158.

In the process of the invention, a particulate catalyst precursormaterial is mixed with a pore modification agent to produce a modifiedcatalyst precursor composition which is formed into a predeterminedshape under compression. As described above, the shaped body into whichthe precursor composition is formed has a volume of at least about 0.02cc, more preferably at least about 0.03 cc, and most preferably, atleast about 0.05 cc. The catalyst precursor composition contained inthis shaped body is transformed into an active catalyst by a series ofsteps conveniently referred to as calcination. This transformation,which is critical for the preparation of superior catalysts, isaccomplished in three stages. For convenience, these may be referred toas (1) initial heat-up stage, (2) rapid heat-up stage, and (3)maintenance/finishing stage. Removal of the pore builder as describedabove may be carried out either prior to or as part of the initialheat-up stage of the calcination.

In the initial heat-up stage, the catalyst precursor is heated in anatmosphere selected from among air, steam, inert gas, and mixturesthereof, at any convenient heat-up rate to a temperature not to exceedthe phase transformation initiation temperature which is about 300° C.In general, suitable temperatures for the initial heat-up stage rangefrom about 200° to about 300° C. with a temperature from about 250° toabout 275° C. being preferred.

If removal of pore builder occurs primarily during this initial heat-upstage, this step should be carried out under the conditions outlinedabove. If the pore builder has been substantially removed prior to theinitial heat-up stage of the transformation to active catalyst, it maystill be desirable to program the introduction of oxygen-containing gasduring the heat-up in order to avoid the possible development of anexcessive exotherm which might prematurely dehydrate the precursor orotherwise adversely affect the activated catalyst. The criteria forexothermic temperature excursions are essentially the same as for thepore builder removal stage, as described hereinabove.

After the desired temperature has been achieved in the initial heat-upstage, the initially selected atmosphere (in the event it does notcontain molecular oxygen and steam and/or has a different compositionthan that which is desired for the rapid heat-up stage) is replaced by amolecular oxygen/steam-containing atmosphere, while maintaining thecatalyst precursor at the temperature achieved in the initial heat-upstage. Such atmosphere optionally may contain an inert gas and, as such,may be conveniently represented by the formula

    (O.sub.2).sub.x (H.sub.2 O).sub.y (IG).sub.z

wherein IG is an inert gas and x, y, and z represent mol % (or volume %)of the O₂, H₂ O, and IG components, respectively, in the molecularoxygen/steam-containing atmosphere, with x having a value greater thanzero (0) mol %, but less than 100 mol %, y having a value greater thanzero (0) mol %, but less than 100 mol %, and z having a valuerepresenting the balance of the molecular oxygen/steam-containingatmosphere. At least during the critical heating period, it ispreferably at least about 5% by volume. A critical feature of theinstant invention is that such atmosphere must contain at least aportion of molecular oxygen and water (as steam). The presence of theinert gas in such atmosphere, as indicated by the formula, is optional.Nonlimiting examples of suitable inert gases suitable for use in themolecular oxygen/steam-containing atmosphere include (molecular)nitrogen, helium, argon, and the like, with nitrogen generally beingpreferred for practical reasons.

Once the molecular oxygen/steam-containing atmosphere is provided, thecatalyst precursor is subjected to the rapid heat-up stage of thecalcination. In the rapid heat-up stage, the initial heat-up stagetemperature of the catalyst is increased at a rate of from about 2° C.per minute (° C./min) to about 12° C./min, preferably from about 4°C./min to about 8° C./min, to a value effective to eliminate or removethe water of hydration from the catalyst precursor. In general, atemperature of from about 340° C. to about 450° C., usually at leastabout 375° C. to about 425° C. is suitable. Because of the importance ofcontrolling the rate of increase during this temperature ramp, it isessential that residual pore builder have been removed to the extentthat it does not result in the generation of an exotherm at temperatureswithin the range of the ramp heating that would cause the rate ofincrease to significantly exceed about 12° C.

Because of the apparent presence of residual pore builder, or carbonresulting from coking of the pore builder during the pore builderremoval step, it is important to control the heatup rate of theprecursor bodies themselves within the aforesaid 2° to 12° C. range. Tokeep the rate of temperature increase within such range, the oven inwhich the calcination takes place is preferably heated at a rate nohigher than about 3° C., more preferably not more than about 2° C. Asnoted above, a heatup rate approaching 12° C. per minute may result in asubstantial exotherm. Where stearic acid is used as the pore builder,the optimal heatup rate is about 1.8° C. per minute. The acceptablelevel of residual pore builder remaining after the pore builder removalstep may be defined with reference to transformation of the precursor toactive catalyst at a standard or reference heating rate of 1.8° C.between 300° and 400° C. A maximum acceptable level of residual porebuilder is that concentration which does not result in an exotherm thatadversely alters or affects the VPO structure when the shaped bodies areheated for activation in the presence of air, steam or nitrogen undersuch reference conditions.

Following the rapid heat-up stage, the catalyst precursor is subjectedto the maintenance/finishing stage of calcination. In themaintenance/finishing stage, the temperature, while maintaining themolecular oxygen/steam-containing atmosphere, is adjusted to a valuegreater than 350° C., but less than 550° C., preferably from about 375°C. to about 450° C., most preferably from about 400° C. to about 425° C.The adjusted temperature is then maintained, first in the molecularoxygen/steam-containing atmosphere for a time effective to provide avanadium oxidation state of from about +4.06 to about +4.3 or simplyfrom about 4.06 to about 4.3, and thereafter in a nonoxidizing,steam-containing atmosphere for a time effective to complete thecatalyst precursor-to-active catalyst transformation to yield the activecatalyst. In a manner similar to the molecular oxygen/steam-containingatmosphere, the nonoxidizing, steam-containing atmosphere alsooptionally may contain an inert gas, with nitrogen generally being thepreferred inert gas for practical reasons.

The nonoxidizing, steam-containing atmosphere need not necessarily becompletely free of molecular oxygen. However, such atmosphere preferablyis substantially free of molecular oxygen. Accordingly, molecular oxygenmay be present in an amount which is not effective to cause furtheroxidation of the vanadium beyond the desired oxidation state of about+4.06 to about +4.3. In general, molecular oxygen may be present inamounts which do not exceed about 0.5 mol % of the nonoxidizing,steam-containing atmosphere.

It will be apparent to those skilled in the art that the period of timeduring which the adjusted temperature is maintained in the molecularoxygen/steam-containing atmosphere in order to provide the desiredvanadium oxidation state of from about +4.06 to about +4.3 will dependto some extent upon the vanadium oxidation state achieved during therapid heat-up stage, which, in turn, will depend to some extent upon theperiod of time during which the catalyst precursor material is exposedto the molecular oxygen/steam-containing atmosphere at the stated rapidheat-up stage temperatures. In general, a period of time of from about0.25 hour to about 4 hours is suitable, with a period of time of fromabout 1 hour to about 2 hours being preferred.

A suitable period of time during which the adjusted temperature ismaintained in the nonoxidizing, steam-containing atmosphere is at least1 hour, although longer periods of time up to 24 hours, or longer, maybe employed, if desired, with a period of time of from about 3 hours toabout 10 hours being preferred, and a period of about 6 hours being mostpreferred.

FIG. 13 illustrates typical profiles of both catalyst body temperatureand oven temperature for the catalyst activation steps of the process ofthe invention. It may be noted that the circulating gas serves as a heatsource in supplying the heat of vaporization during the pore builderremoval step, but functions as a heat sink during the activation stepwhen residual pore builder is oxidized. The circulating gas may alsoserve as a heat sink whenever a modest exotherm is incurred in removalof pore builder, for example, as illustrated in FIG. 5.

In accordance with the invention, it has been discovered that both theremoval of pore builder and the transformation of catalyst precursorbodies to active catalyst bodies can be conducted in the same oven,without removal of the catalyst bodies from the oven between the endpoint of the pore builder removal step and the beginning of thetransformation step. As indicated above, the end point of the porebuilder removal step is determined by the residual concentration of porebuilder, and/or carbonaceous degradation products of the pore builder,which must be reduced to such a level that no excessive exotherm isexperienced in the subsequent activation step. To assure against such anexotherm, it is necessary that at least 80% by weight of the porebuilder originally present be removed from the precursor bodies byraising the terminal temperature of the precursor bodies at the endpoint to at least about 200° C., preferably in the range of 240° to 260°C. Advantageously, the precursor bodies are not cooled below about 100°C. in the period between the end point of the removal step and thebeginning of the activation step. More preferably, the precursor bodiesare not allowed to cool by more than 50° C. during that period. Usingthis preferred embodiment of the process of the invention, enhancedproductivity is achieved by avoiding the steps of cooling the precursorbodies, transferring them from one oven to another, and reheating themin the first step of the activation process. Substantial energy savingsare also realized.

Catalysts of the present invention have been demonstrated to affordenhanced yields in the range of 2-4% higher than otherwise comparablecatalysts that have not been processed to provide the proportions ofmacropores described hereinabove. The catalysts of the invention arealso superior to catalysts which contain a similar proportion ofmacropores, but have been prepared by burning a pore builder out of acatalyst precursor tablet or pellet under conditions which expose thecatalyst body to a substantial exotherm. Careful removal of the porebuilder under mild conditions avoids the deposit of carbon on the activesurfaces of the catalyst pores or abstraction of labile oxygens andreduction of the vanadium oxidation state, and further avoidsinterference with the critical chemistry by which the precursor istransformed to high surface area active catalyst under the controlledheating conditions described above.

FIGS. 8-12 are photomicrographs showing the external surface of acatalyst tablet of the invention. Distributed over the surface of thistablet are exit holes through which the pore builder has escaped duringthe process of its removal. By removal of the pore builder under mildconditions, a distribution is obtained in which the exit holes greaterthan about 2 microns in diameter have a density of at least about75/mm², more preferably at least about 100/mm² on the external surfacesof the catalyst bodies. These holes are in communication with theinterior of the body and further facilitate the ingress of reactantgases and egress of product gases.

The following examples illustrate the invention.

EXAMPLE 1

VPO catalyst precursor was prepared by reaction of vanadium oxide andphosphoric acid in an isobutyl alcohol medium as described in U.S. Pat.No. 5,275,996. The precipitated precursor was separated from the alcoholmedium and dried in a roaster. Dry precursor powder (1500 pounds)containing up to 25% precursor powder dust recycled from downstreamslugging and tableting operations was blended in a cone blender withgraphite (4% by weight based on the final weight of the blend) andstearic acid (10% by weight based on the final weight of the blend)having an average particle diameter of about 43 μM. Since the recycledust contained graphite and stearic acid in these proportions, freshgraphite was charged in a proportion of 4 wt % based on the total weightof fresh materials, and fresh stearic acid was charged in a proportionof 10 wt % based on the total weight of fresh materials. The freshcatalyst precursor, recycle dust, graphite and stearic acid componentsof the blender charge were weighed in drums and the transferredpneumatically to the cone blender. Once the charge was complete, thevessel was rotated end over end for approximately one hour to provideadequate mixing of the ingredients. Thereafter, the blended powder wastransferred into feed hoppers that held approximately 1000 pounds each.The blend was transferred from these hoppers to the tableting operation.

The powder blend was prepared for tableting by preliminary slugging andgranulating. The powder was first transferred by a screw conveyor to afeed charger ("feed frame") from which the powder was charged to aslugging press. Slug dies were rotated under the feed frame and filledby gravity flow of powder from the feed frame. Each die comprised acylinder having piston-like punch in the bottom thereof for removal ofthe slug after compression. As the dies rotated away from the feedframe, the amount of material in each die was adjusted as necessary byraising the height of the lower punch slightly to cause excess powder tooverflow the die. An upper punch was then pressed into the die tocompress the powder in each die into a cylinder of approximately 0.25in. in height and 0.75 in. in diameter. These slugs had a density ofapproximately 1.40 to 1.50 g/cc. Higher densities were avoided becausecompression to higher densities may cause expression of liquid fattyacid from certain grades of stearic acid, resulting in a change in thesize and number of stearic particles.

The slugs were removed from the dies onto a conveyor on which they weretransferred to a Jacobson mill. In the Jacobson mill the slugs weregranulated via mechanical action of knives and exited the mill throughan associated retaining screen. The particle size of the granulesproduced may be varied by varying the rotational speed of the mill. Ingranulating the slugs of this example, the mill was run at a speed of1200 to 1500 rpm, and a retaining screen having 3/32" holes was used.

Tablets were produced from the granulated slugs using a tableting pressgenerally similar to the slugging press. Cylindrical tablets produced onthe tableting press had a diameter of about 0.25", a length of0.210±0.002 in., and a weight of 0.174 to 0.180 g. The tablets werecollected in drum for transfer to the pore builder removal process,sometimes referred to hereinafter as "dewaxing."

Dewaxing was carried using apparatus of the type illustrated in FIGS. 2and 3. Weighed portions of precursor tablets (13 pounds each) wereplaced into trays, each tray having a length of 29" and a width of 24".The bed of tablets thus produced in the trays had a height of about 1".These trays were placed in the oven on racks having vertically spacedsupports slots to hold 19 vertically arrayed trays with a gap of about1.5" between trays; and heated air was circulated over the trays forstripping of the stearic acid pore builder from the precursor. The rackswere arranged to hold two parallel arrays (stacks) of vertically spacedtrays, one stack adjacent the air inlet ("upwind") side of the rack, theother stack adjacent the air outlet ("downwind") side. In the verticalarray on the inlet side, a tray was placed on each support slot, 19trays in all. On the outlet side, trays were placed in every other slot,a total of 9, with a gap of 4" between trays. This arrangement providedthe most advantageous pressure drop balance inside the dewax oven and,thus, the optimal air flow around the trays.

As illustrated in FIGS. 2 and 3, two racks of trays were placed inoven 1. The portion of the oven chamber accommodating each rack wasapproximately 5'×5'×5'. The racks were oriented with their exit sidesadjacent each other and their inlet sides at opposite ends of the oven.By operation of a blower, air was circulated through the oven from bothends, flowing across the trays on the racks in the "tunnels," into acentral plenum chamber at the common exit location of the racks and backthrough a heat exchanger to the suction of the blower. Total air flowwas 9000 scfm, 4500 scfm across each rack. Linear flow velocity wasabout 100 to 150 ft/min. at ambient conditions across each tray. Aircontaining vaporized pore builder was purged from the circulating airstream at a rate of 1000 scfm, and this was balanced by flow of 1000scfm fresh air into the loop. The circulating air was heated by transferof heat from hot air in the indirect heat exchanger. Purge air wasdirected to a thermal oxidizer where the stearic acid vapor was burnedto carbon dioxide and water vapor.

The temperature of the circulating air was slowly increased to heat theprecursor tablets at a rate of 2° C. per minute to a temperature of 165°C. The temperature was held constant at 165° C. for 2 hours. Then, theprecursor was heated from 165° C. to a first intermediate temperature of199° C. at a rate of 2° C. per hour, and from 199° C. to a terminaltemperature of 255° C. at a rate of 28° C. per hour. Thereafter, theprecursor bodies were cooled to ambient temperature in about 1 hour. Attemperatures above about 170° C., stearic acid is catalytically oxidizedin the presence of the precursor composition, and a controlled burnofftakes place.

After precursor bodies had cooled, they were tested by differentialscanning calorimetry for verification that the stearic acid had beenessentially completely removed. Once that verification was established,the trays of precursor bodies were transferred to another oven for heattreatment to transform the precursor composition to active catalyst. Theheat treatment oven was a single chamber unit similar in design to thedewax oven but adapted for direct heating of the atmosphere within theoven and capable of operation at much higher temperatures. As in thedewaxing oven, the trays of precursor bodies were placed on racks inwhich the trays were arrayed vertically with a gap of 1.5" betweentrays. The oven accommodated two racks of trays, with 38 trays on eachrack. Unlike the racks used in the dewaxing oven, the racks used fortransformation of the precursor were open to gas flow from alldirections. Transformation of the precursor to active catalyst wascarried out substantially in the manner described in U.S. Pat. No.5,137,860. However, apparently due to small residual amounts of stearicacid in the pores of the precursor bodies, it was necessary to increasethe temperature in the heat treatment oven at a rate somewhat slowerthan that found optimal for catalyst produced without a poremodification agent. Thus, the atmosphere within the oven was heated at arate of only about 1.8° C. per minute, resulting in a 3° to 9° C. perminute increase in the temperature of the precursor bodies themselves.

After the activation step was completed, the pore size distribution ofthe catalyst was determined by the standard mercury intrusionporosimetry test. To provide a basis of comparison, the same porosimetrytest was run on a catalyst which was prepared in substantially the samemanner as the catalyst of this example, except that no pore modificationagent was used in the preparation of the precursor shaped bodies.Results of the porosimetry tests are set forth in Tables 1 and 2. Plotsof the pore size distributions of the two catalysts as measured bymercury intrusion are shown illustrated in FIG. 1.

                  TABLE 1                                                         ______________________________________                                        Pore                                                                          Diameter                                                                             Incremental                                                                              Per Cent     Per Cent                                       (microns)                                                                            Volume (ml/g)                                                                            Incremental Volume                                                                         Cummulative Volume                             ______________________________________                                        300.0000                                                                             0.0000     0.00         0.00                                           179.2623                                                                             0.0005     0.19         0.19                                           143.5927                                                                             0.0001     0.04         0.23                                           119.7625                                                                             0.0002     0.08         0.31                                           86.1337                                                                              0.0004     0.16         0.47                                           67.2903                                                                              0.0004     0.16         0.62                                           51.6881                                                                              -0.0001    -0.04        0.58                                           40.4661                                                                              0.0003     0.12         0.70                                           31.3759                                                                              0.0003     0.12         0.82                                           24.5693                                                                              0.0001     0.04         0.85                                           19.1229                                                                              0.0001     0.04         0.89                                           14.9069                                                                              0.0004     0.16         1.05                                           12.3683                                                                              0.0001     0.04         1.09                                           9.0387 0.0004     0.16         1.24                                           7.0202 0.0007     0.27         1.52                                           6.0480 0.0002     0.08         1.59                                           4.2846 0.0005     0.19         1.79                                           3.3326 0.0007     0.27         2.06                                           2.5857 0.0006     0.23         2.29                                           2.0157 0.0012     0.47         2.76                                           1.5590 0.0015     0.58         3.34                                           1.2188 0.0028     1.09         4.43                                           0.9478 0.0030     1.17         5.59                                           0.7400 0.0039     1.52         7.11                                           0.5750 0.0053     2.06         9.17                                           0.4494 0.0065     2.53         11.69                                          0.3488 0.0060     2.33         14.02                                          0.2720 0.0065     2.53         16.55                                          0.2123 0.0064     2.49         19.04                                          0.1644 0.0062     2.41         21.45                                          0.1282 0.0112     4.35         25.80                                          0.1003 0.0200     7.77         33.57                                          0.0781 0.0392     15.23        48.80                                          0.0628 0.0397     15.42        64.22                                          0.0604 0.0012     0.47         64.69                                          0.0473 0.0365     14.18        78.87                                          0.0369 0.0177     6.88         85.74                                          0.0313 0.0049     1.90         87.65                                          0.0286 0.0029     1.13         88.77                                          0.0224 0.0121     4.70         93.47                                          0.0174 0.0042     1.63         95.10                                          0.0136 0.0037     1.44         96.54                                          0.0106 0.0016     0.62         97.16                                          0.0082 0.0018     0.70         97.86                                          0.0064 0.0014     0.54         98.41                                          0.0050 0.0012     0.47         98.87                                          0.0039 0.0014     0.54         99.42                                          0.0031 0.0015     0.56         100.00                                         ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Prior Art                                                                     Pore   Prior Art  Prior Art    Prior Art                                      Diameter                                                                             Incremental                                                                              Per Cent     Per Cent                                       (microns)                                                                            Volume (ml/g)                                                                            Incremental Volume                                                                         Cummulative Volume                             ______________________________________                                        393.6248                                                                             0.0000     0.00         0.00                                           288.7924                                                                             0.0017     0.75         0.75                                           131.8612                                                                             0.0002     0.09         0.84                                           103.0141                                                                             0.0010     0.44         1.28                                           76.7579                                                                              0.0002     0.09         1.37                                           59.5883                                                                              0.0002     0.09         1.45                                           46.1673                                                                              0.0001     0.04         1.50                                           35.8920                                                                              0.0007     0.31         1.81                                           27.9914                                                                              0.0002     0.09         1.89                                           21.8714                                                                              0.0001     0.04         1.94                                           17.0155                                                                              0.0002     0.09         2.03                                           13.6331                                                                              0.0003     0.13         2.16                                           10.7045                                                                              0.0001     0.04         2.20                                           8.0368 0.0002     0.09         2.29                                           6.5403 0.0003     0.13         2.42                                           5.0966 0.0004     0.18         2.60                                           3.7449 0.0001     0.04         2.64                                           2.9684 0.0000     0.00         2.64                                           2.3087 0.0001     0.04         2.69                                           1.7974 0.0001     0.04         2.73                                           1.3969 0.0002     0.09         2.82                                           1.0866 0.0001     0.04         2.86                                           0.8470 0.0002     0.09         2.95                                           0.6594 0.0001     0.04         2.99                                           0.5128 0.0001     0.04         3.04                                           0.3977 0.0001     0.04         3.08                                           0.3087 0.0007     0.31         3.39                                           0.2417 0.0010     0.44         3.83                                           0.1876 0.0031     1.37         5.20                                           0.1455 0.0083     3.65         8.85                                           0.1144 0.0228     10.04        18.89                                          0.0893 0.0401     17.66        36.55                                          0.0704 0.0299     13.17        49.71                                          0.0616 0.0019     0.84         50.55                                          0.0540 0.0291     12.81        63.36                                          0.0422 0.0179     7.88         71.25                                          0.0341 0.0068     2.99         74.24                                          0.0300 0.0040     1.76         76.00                                          0.0255 0.0151     8.65         82.65                                          0.0199 0.0087     3.83         86.48                                          0.0155 0.0084     2.70         90.18                                          0.0121 0.0054     2.38         92.56                                          0.0094 0.0047     2.07         94.63                                          0.0073 0.0037     1.63         96.26                                          0.0057 0.0031     1.37         97.62                                          0.0044 0.0027     1.19         98.81                                          0.0035 0.0027     1.19         100.00                                         ______________________________________                                    

EXAMPLES 2-9

A series of catalysts was prepared substantially in the manner describedin Example 1. Set forth in Table 3 are the time and temperatureconditions under which the pore builder was removed from the catalystprecursors of these Examples, and the ramp catalyst body heating rateduring catalyst activation.

                  TABLE 3                                                         ______________________________________                                             R.T. to                                                                       Hold    Hold    Hold  Dewax Schedule                                                                            Activation                             Batch                                                                              Temp.   Temp.   Period                                                                              After Hold Period                                                                         Ramp Rate                              ______________________________________                                         6.sup.a                                                                           1 h     165° C.                                                                        2 h   165° C.-199° C.; 17                                                         1.8° C./h                                                  199° C.-255° C.; 2 h                  7.sup.b                                                                           1 h     165° C.                                                                        2 h   165° C.-199° C.; 17                                                         1.8° C./h                                                  199° C.-255° C.; 2 h                  9.sup.c                                                                           1 h     165° C.                                                                        2 h   165° C.-199° C.; 17                                                         1.8° C./h                                                  199° C.-255° C.; 2 h                 21.sup.d                                                                           1 h     165° C.                                                                        2 h   165° C.-199° C.; 17                                                         1.8° C./h                                                  199° C.-255° C.; 2 h                 22.sup.e                                                                           1 h     165° C.                                                                        2 h   165° C.-199° C.; 17                                                         1.8° C./h                                                  199° C.-255° C.; 2 h                 23.sup.f                                                                           1 h     165° C.                                                                        2 h   165° C.-199° C.; 17                                                         1.8° C./h                                                  199° C.-255° C.; 2 h                 24.sup.g                                                                           1 h     165° C.                                                                        2 h   165° C.-199° C.; 17                                                         1.8° C./h                                                  199° C.-255° C.; 2 h                 25.sup.h                                                                           1 h     165° C.                                                                        2 h   165° C.-199° C.; 17                                                         1.8° C./h                                                  199° C.-255° C.; 2                   ______________________________________                                                                   h                                                   .sup.a Example 2                                                              .sup.b Example 3                                                              .sup.c Example 4                                                              .sup.d Example 5                                                              .sup.e Example 6                                                              .sup.f Example 7                                                              .sup.g Example 8                                                              .sup.h Example 9                                                         

For each of the activated catalysts of these Examples, the pore volumedistribution was determined by mercury porosimetry, the vanadiumoxidation state was determined by chemical titration, and the surfacearea was determined by the B.E.T. method. The dimensions, % "brokenpills" (B.P.(%)), wt % minus 10 mesh particles (%-10M), bulk density(B.D.) (g/cc), B.E.T. surface area (m² /g), vanadium oxidation state,and phosphorus to vanadium ratios for the catalysts of these examplesare set forth in Table 4. Results of the porosimetry tests are set forthin Tables 5 to 11.

                                      TABLE 4                                     __________________________________________________________________________    PROPERTIES OF ANST BATCHES                                                         Batch                                                                             Crush                                                                             Weight                                                                            Length                                                                            B.P.                                                                             -10 Mesh                                                                           B.D.                                                                             S.A.                                          Example                                                                            Number                                                                            (lbs.)                                                                            (grams)                                                                           (inches)                                                                          (%)                                                                              (%)  (g/cc)                                                                           (m.sup.2 /g)                                                                      VOX P/V                                   __________________________________________________________________________    2     6  8.5 0.131                                                                             0.200                                                                             0.34                                                                             0.34 0.631                                                                            22  4.21                                                                              1.071                                 3     7  8.4 0.136                                                                             0.203                                                                             0.18                                                                             0.37 0.615                                                                            19  4.14                                                                              1.084                                 4     9  9.2 0.133                                                                             0.200                                                                             0.43                                                                             0.49 0.614                                                                            23  4.19                                                                              1.085                                 5    21  10.1                                                                              0.139                                                                             0.204                                                                             0.94                                                                             0.16 0.667                                                                            17  4.16                                                                              1.078                                 6    22  7.8 0.139                                                                             0.203                                                                             0.74                                                                             0.12 0.646                                                                            14  4.16                                                                              1.075                                 7    23  10.3                                                                              0.145                                                                             0.204                                                                             0.61                                                                             0.20 0.649                                                                            23  4.18                                                                              1.086                                 8    24  8.3 0.142                                                                             0.205                                                                             0.53                                                                             0.14 0.643                                                                            22  4.18                                                                              1.088                                 9    25  11.5                                                                              0.143                                                                             0.208                                                                             0.49                                                                             0.35 0.650                                                                            18  4.17                                                                              1.077                                 Average  8.23                                                                              0.123                                                                             0.181                                                                             0.473                                                                            0.241                                                                              0.568                                                                            17.6                                                                              4.17                                                                              1.080                                 Standard Deviation                                                                     3.117                                                                             0.044                                                                             0.064                                                                             0.267                                                                            0.146                                                                              0.202                                                                            6.849                                                                             0.020                                                                             0.006                                 __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________    Excel DATA 1.107 (Batch 6)                                                    PORE     Incremental                                                                         Incremental                                                                         Cummulative  PORE  Incremental                                                                         Incremental                     DIAMETER Intrusion                                                                           intrustion                                                                          Intrusion    DIAMETER                                                                            Intrusion                                                                           Intrusion                       μm    mL/g  Per Cent                                                                            Per Cent     μm mL/g  Per Cent                        __________________________________________________________________________       300   0     0.00  0.00                                                        179.2623                                                                            0.0005                                                                              0.19  0.19                                                        143.5927                                                                            1E-04 0.04  0.23                                                        119.7625                                                                            0.0002                                                                              0.08  0.31                                                        86.1337                                                                             0.0004                                                                              0.16  0.47                                                        67.2903                                                                             0.0004                                                                              0.16  0.62                                                        51.6881                                                                             -0.0001                                                                             -0.04 0.58                                                        40.4661                                                                             0.0003                                                                              0.12  0.70                                                        31.3759                                                                             0.0003                                                                              0.12  0.82                                                        24.5693                                                                             0.0001                                                                              0.04  0.85                                                        19.1229                                                                             1E-04 0.04  0.89                                                        14.9069                                                                             0.0004                                                                              0.16  1.05                                                        12.3683                                                                             1E-04 0.04  1.09                                                        9.0387                                                                              0.0004                                                                              0.16  1.24                                                        7.0202                                                                              0.0007                                                                              0.27  1.52                                                        6.048 0.0002                                                                              0.08  1.59                                                        4.2846                                                                              0.0005                                                                              0.19  1.79                                                        3.3326                                                                              0.0007                                                                              0.27  2.06         3.3326                                                                              0.0007                                                                              0.27                               2.5857                                                                              0.0006                                                                              0.23  2.29         2.5857                                                                              0.0006                                                                              0.23                               2.0157                                                                              0.0012                                                                              0.47  2.76         2.0157                                                                              0.0012                                                                              0.47                               1.559 0.0015                                                                              0.58  3.34         1.559 0.0015                                                                              0.58                               1.2188                                                                              0.0028                                                                              1.09  4.43         1.2188                                                                              0.0028                                                                              1.09                               0.9478                                                                              0.003 1.17  5.59         0.9478                                                                              0.003 1.17                               0.74  0.0039                                                                              1.52  7.11         0.74  0.0039                                                                              1.52                               0.575 0.0053                                                                              2.06  9.17         0.575 0.0053                                                                              2.06                               0.4494                                                                              0.0065                                                                              2.53  11.69        0.4494                                                                              0.0065                                                                              2.53                               0.3488                                                                              0.006 2.33  14.02        0.3488                                                                              0.006 2.33                               0.272 0.0065                                                                              2.53  16.55        0.272 0.0065                                                                              2.53                               0.2123                                                                              0.0064                                                                              2.49  19.04        0.2123                                                                              0.0064                                                                              2.49                               0.1644                                                                              0.0062                                                                              2.41  21.45        0.1644                                                                              0.0062                                                                              2.41                               0.1282                                                                              0.0112                                                                              4.35  25.80        0.1282                                                                              0.0112                                                                              4.35                               0.1003                                                                              0.02  7.77  33.57        0.1003                                                                              0.002 7.77                               0.0781                                                                              0.0392                                                                              15.23 48.80  total       0.0818                                                                              31.78                              0.0628                                                                              0.0397                                                                              15.42 64.22  double check                                                                              31.78 31.78                              0.0604                                                                              0.0012                                                                              0.47  64.69  answer is   31.78                                    0.0473                                                                              0.0365                                                                              14.18 78.87                                                       0.0369                                                                              0.0177                                                                              6.88  85.74                                                       0.0313                                                                              0.0049                                                                              1.90  87.65                                                       0.0286                                                                              0.0029                                                                              1.13  88.77                                                       0.0224                                                                              0.01212                                                                             4.70  93.47                                                       0.0174                                                                              0.0042                                                                              1.63  95.10                                                       0.0136                                                                              0.0037                                                                              1.44  96.54                                                       0.0106                                                                              0.0016                                                                              0.62  97.16                                                       0.0082                                                                              0.0018                                                                              0.70  97.86                                                       0.0064                                                                              0.0014                                                                              0.54  98.41                                                       0.005 0.0012                                                                              0.47  98.87                                                       0.0039                                                                              0.0014                                                                              0.54  99.42                                                       0.0031                                                                              0.0015                                                                              0.58  100.00                                                   total    0.2574                                                                              100.00                                                         __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    Excel DATA 1.110 (Batch 7)                                                    PORE     Incremental                                                                         Incremental                                                                         Cummulative  PORE  Incremental                                                                         Incremental                     DIAMETER Intrusion                                                                           intrustion                                                                          Intrusion    DIAMETER                                                                            Intrusion                                                                           Intrusion                       μm    mL/g  Per Cent                                                                            Per Cent     μm mL/g  Per Cent                        __________________________________________________________________________       300   0     0.00  0.00                                                        179.5482                                                                            0.0009                                                                              0.26  0.26                                                        143.7761                                                                            0.0008                                                                              0.23  0.50                                                        120.0178                                                                            0.0006                                                                              0.18  0.67                                                        85.8709                                                                             0.0006                                                                              0.18  0.85                                                        67.21 0.0001                                                                              0.03  0.88                                                        51.7832                                                                             0.0007                                                                              0.20  1.08                                                        40.3935                                                                             0.0003                                                                              0.09  1.17                                                        31.3584                                                                             0.0001                                                                              0.03  1.20                                                        24.6069                                                                             0     0.00  1.20                                                        19.1229                                                                             0.0005                                                                              0.15  1.35                                                        14.897                                                                              0.0002                                                                              0.06  1.41                                                        12.3643                                                                             0.0004                                                                              0.12  1.52                                                        9.0423                                                                              0.0003                                                                              0.09  1.61                                                        7.0207                                                                              0.0005                                                                              0.15  1.76                                                        6.0483                                                                              0.0008                                                                              0.23  1.99                                                        4.2406                                                                              0.0006                                                                              0.18  2.17                                                        3.3129                                                                              0.0012                                                                              0.35  2.52         3.3129                                                                              0.0012                                                                              0.35                               2.5791                                                                              0.0023                                                                              0.67  3.19         2.5791                                                                              0.0023                                                                              0.67                               2.0118                                                                              0.0024                                                                              0.70  3.89         2.0118                                                                              0.0024                                                                              0.70                               1.5654                                                                              0.0052                                                                              1.52  5.42         1.5654                                                                              0.0052                                                                              1.52                               1.2172                                                                              0.0063                                                                              1.84  7.26         1.2172                                                                              0.0063                                                                              1.84                               0.9498                                                                              0.0092                                                                              2.69  9.95         0.9498                                                                              0.0092                                                                              2.69                               0.7408                                                                              0.0107                                                                              3.13  13.09        0.7408                                                                              0.0107                                                                              3.13                               0.5725                                                                              0.0113                                                                              3.31  16.39        0.5725                                                                              0.0113                                                                              3.31                               0.4494                                                                              0.0098                                                                              2.87  19.26        0.4494                                                                              0.0098                                                                              2.87                               0.3502                                                                              0.0075                                                                              2.20  21.46        0.3502                                                                              0.0075                                                                              2.20                               0.2726                                                                              0.0067                                                                              1.96  23.42        0.2726                                                                              0.0067                                                                              1.96                               0.2117                                                                              0.0062                                                                              1.81  25.23        0.2117                                                                              0.0062                                                                              1.81                               0.1646                                                                              0.0094                                                                              2.75  27.99        0.1646                                                                              0.0094                                                                              2.75                               0.1286                                                                              0.0184                                                                              5.39  33.37        0.1286                                                                              0.0184                                                                              5.39                               0.1003                                                                              0.0333                                                                              9.75  43.12        0.1003                                                                              0.0333                                                                              9.75                               0.0779                                                                              0.0436                                                                              12.76 55.88  total       0.1399                                                                              40.95                              0.0627                                                                              0.0339                                                                              9.92  65.81  double check                                                                              40.95 40.95                              0.0605                                                                              0.0015                                                                              0.44  66.25  answer is   40.95                                    0.0474                                                                              0.0345                                                                              10.10 76.35                                                       0.0369                                                                              0.0182                                                                              5.33  81.67                                                       0.0314                                                                              0.0074                                                                              2.17  83.84                                                       0.0287                                                                              0.004 1.17  85.01                                                       0.0224                                                                              0.0146                                                                              4.27  89.29                                                       0.0174                                                                              0.0069                                                                              2.02  91.31                                                       0.0136                                                                              0.0064                                                                              1.87  93.18                                                       0.0106                                                                              0.004 1.17  94.35                                                       0.0082                                                                              0.0036                                                                              1.05  95.40                                                       0.0064                                                                              0.0041                                                                              1.20  96.60                                                       0.005 0.0036                                                                              1.05  97.66                                                       0.0039                                                                              0.004 1.17  98.83                                                       0.0031                                                                              0.004 1.17  100.00                                                   total    0.3416                                                                              100.00                                                         __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    Excel DATA 1.112 (Batch 9)                                                    PORE     Incremental                                                                         Incremental                                                                         Cummulative  PORE  Incremental                                                                         Incremental                     DIAMETER Intrusion                                                                           intrustion                                                                          Intrusion    DIAMETER                                                                            Intrusion                                                                           Intrusion                       μm    mL/g  Per Cent                                                                            Per Cent     μm mL/g  Per Cent                        __________________________________________________________________________       300   0     0.00  0.00                                                        178.1277                                                                            0.0011                                                                              0.33  0.33                                                        142.6828                                                                            0.0009                                                                              0.27  0.60                                                        119.3815                                                                            0.0004                                                                              0.12  0.72                                                        86.1996                                                                             0.0003                                                                              0.09  0.81                                                        66.9303                                                                             0.0002                                                                              0.06  0.87                                                        51.8787                                                                             0.0005                                                                              0.15  1.02                                                        40.2779                                                                             0.0003                                                                              0.09  1.11                                                        31.3497                                                                             0.0003                                                                              0.09  1.20                                                        24.5962                                                                             0.0003                                                                              0.09  1.29                                                        19.1197                                                                             0.0007                                                                              0.21  1.50                                                        14.895                                                                              0.0002                                                                              0.06  1.56                                                        12.3548                                                                             0.0004                                                                              0.12  1.69                                                        9.0336                                                                              0.0003                                                                              0.09  1.78                                                        7.036 0.0004                                                                              0.12  1.90                                                        6.0353                                                                              0.0006                                                                              0.18  2.08                                                        4.2653                                                                              0.001 0.30  2.38                                                        3.3259                                                                              0.001 0.30  2.68         3.3259                                                                              0.001 0.30                               2.5889                                                                              0.0029                                                                              0.87  3.55         2.5889                                                                              0.0029                                                                              0.87                               2.0173                                                                              0.0041                                                                              1.23  4.78         2.0173                                                                              0.0041                                                                              1.23                               1.5655                                                                              0.0071                                                                              2.14  6.92         1.5655                                                                              0.0071                                                                              2.14                               1.2166                                                                              0.0092                                                                              2.77  9.69         1.2166                                                                              0.0092                                                                              2.77                               0.9482                                                                              0.0106                                                                              3.19  12.88        0.9482                                                                              0.0106                                                                              3.19                               0.7418                                                                              0.0116                                                                              3.49  16.37        0.7418                                                                              0.0116                                                                              3.49                               0.5762                                                                              0.0104                                                                              3.13  19.50        0.5762                                                                              0.0104                                                                              3.13                               0.4495                                                                              0.0084                                                                              2.53  22.03        0.4495                                                                              0.0084                                                                              2.53                               0.3502                                                                              0.0066                                                                              1.99  24.01        0.3502                                                                              0.0066                                                                              1.99                               0.2723                                                                              0.0056                                                                              1.69  25.70        0.2723                                                                              0.0056                                                                              1.69                               0.2124                                                                              0.0058                                                                              1.75  27.45        0.2124                                                                              0.0058                                                                              1.75                               0.1636                                                                              0.0079                                                                              2.38  29.82        0.1636                                                                              0.0079                                                                              2.38                               0.1287                                                                              0.0151                                                                              4.54  34.37        0.1287                                                                              0.0151                                                                              4.54                               0.1001                                                                              0.0367                                                                              11.04 45.41        0.1001                                                                              0.0367                                                                              11.04                              0.0781                                                                              0.0401                                                                              12.07 57.48  total       0.143 43.03                              0.0626                                                                              0.0314                                                                              9.45  66.93  double check                                                                              43.03 43.03                              0.0605                                                                              0.0005                                                                              0.15  67.08  answer is   43.03                                    0.0474                                                                              0.029 8.73  75.80                                                       0.0369                                                                              0.0178                                                                              5.36  81.16                                                       0.0313                                                                              0.0053                                                                              1.59  82.76                                                       0.0287                                                                              0.004 1.20  83.96                                                       0.0224                                                                              0.0148                                                                              4.45  88.41                                                       0.0174                                                                              0.008 2.41  90.82                                                       0.0136                                                                              0.0071                                                                              2.14  92.96                                                       0.0106                                                                              0.0046                                                                              1.38  94.34                                                       0.0082                                                                              0.0033                                                                              0.99  95.34                                                       0.0064                                                                              0.0045                                                                              1.35  96.69                                                       0.005 0.0026                                                                              0.78  97.47                                                       0.0039                                                                              0.0039                                                                              1.17  98.65                                                       0.0031                                                                              0.0045                                                                              1.35  100.00                                                   total    0.3323                                                                              100.00                                                         __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    Excel DATA 1.123 (Batch 21)                                                   PORE     Incremental                                                                         Incremental                                                                         Cummulative  PORE  Incremental                                                                         Incremental                     DIAMETER Intrusion                                                                           intrustion                                                                          Intrusion    DIAMETER                                                                            Intrusion                                                                           Intrusion                       μm    mL/g  Per Cent                                                                            Per Cent     μm mL/g  Per Cent                        __________________________________________________________________________       300   0     0.00  0.00                                                        177.5658                                                                            0.0007                                                                              0.27  0.27                                                        143.96                                                                              0.0003                                                                              0.11  0.38                                                        120.1459                                                                            0.0002                                                                              0.08  0.46                                                        85.6097                                                                             0.0008                                                                              0.31  0.77                                                        66.9303                                                                             0.0003                                                                              0.11  0.88                                                        51.7832                                                                             0.0006                                                                              0.23  1.11                                                        40.408                                                                              0.0001                                                                              0.04  1.15                                                        31.3497                                                                             1E-04 0.04  1.19                                                        24.564                                                                              0.0005                                                                              0.19  1.38                                                        19.1294                                                                             0.0008                                                                              0.31  1.68                                                        14.8931                                                                             0.0003                                                                              0.11  1.80                                                        12.3683                                                                             0.0004                                                                              0.15  1.95                                                        9.0105                                                                              0.0006                                                                              0.23  2.18                                                        7.036 0.0009                                                                              0.34  2.53                                                        6.0363                                                                              0.001 0.38  2.91                                                        4.2586                                                                              -0.0006                                                                             -0.23 2.68                                                        3.3261                                                                              0.0017                                                                              0.65  3.33         3.3261                                                                              0.0017                                                                              0.65                               2.5893                                                                              0.0034                                                                              1.30  4.63         2.5893                                                                              0.0034                                                                              1.30                               2.0137                                                                              0.0045                                                                              1.72  6.35         2.0137                                                                              0.0045                                                                              1.72                               1.5677                                                                              0.0046                                                                              1.76  8.11         1.5677                                                                              0.0046                                                                              1.76                               1.2171                                                                              0.0053                                                                              2.03  10.14        1.2171                                                                              0.0053                                                                              2.03                               0.9496                                                                              0.0069                                                                              2.64  12.78        0.9496                                                                              0.0069                                                                              2.64                               0.7418                                                                              0.0084                                                                              3.21  16.00        0.7418                                                                              0.0084                                                                              3.21                               0.5757                                                                              0.0082                                                                              3.14  19.14        0.5757                                                                              0.0082                                                                              3.14                               0.4477                                                                              0.0074                                                                              2.83  21.97        0.4477                                                                              0.0074                                                                              2.83                               0.3499                                                                              0.0054                                                                              2.07  24.03        0.3499                                                                              0.0054                                                                              2.07                               0.2706                                                                              0.0048                                                                              1.84  25.87        0.2706                                                                              0.0048                                                                              1.84                               0.2121                                                                              0.0044                                                                              1.68  27.55        0.2121                                                                              0.0044                                                                              1.68                               0.1632                                                                              0.0058                                                                              2.22  29.77        0.1632                                                                              0.0058                                                                              2.22                               0.1282                                                                              0.0084                                                                              3.21  32.99        0.1282                                                                              0.0084                                                                              3.21                               0.1003                                                                              0.0168                                                                              6.43  39.42        0.1003                                                                              0.0168                                                                              6.43                               0.0781                                                                              0.0243                                                                              9.30  48.72  total       0.096 36.74                              0.0628                                                                              0.0239                                                                              9.15  57.86  double check                                                                              38.74 36.74                              0.0604                                                                              0.0006                                                                              0.23  58.09  answer is   36.74                                    0.0474                                                                              0.0266                                                                              10.18 68.27                                                       0.0369                                                                              0.0209                                                                              8.00  76.27                                                       0.0313                                                                              0.0074                                                                              2.83  79.10                                                       0.0287                                                                              0.005 1.91  81.02                                                       0.0224                                                                              0.0165                                                                              6.31  87.33                                                       0.0174                                                                              0.0079                                                                              3.02  90.36                                                       0.0136                                                                              0.0075                                                                              2.87  93.23                                                       0.0106                                                                              0.0035                                                                              1.34  94.57                                                       0.0082                                                                              0.0037                                                                              1.42  95.98                                                       0.0064                                                                              0.0031                                                                              1.19  97.17                                                       0.005 0.0021                                                                              0.80  97.97                                                       0.0039                                                                              0.0025                                                                              0.96  98.93                                                       0.0031                                                                              0.0028                                                                              1.07  100.00                                                   total    0.2613                                                                              100.00                                                         __________________________________________________________________________

                                      TABLE 9                                     __________________________________________________________________________    Excel DATA 1.126 (Batch 22)                                                   PORE     Incremental                                                                         Incremental                                                                         Cummulative  PORE  Incremental                                                                         Incremental                     DIAMETER Intrusion                                                                           intrustion                                                                          Intrusion    DIAMETER                                                                            Intrusion                                                                           Intrusion                       μm    mL/g  Per Cent                                                                            Per Cent     μm mL/g  Per Cent                        __________________________________________________________________________       300   0     0.00  0.00                                                        179.835                                                                             0.0005                                                                              0.22  0.22                                                        144.5144                                                                            0.0003                                                                              0.13  0.34                                                        118.8772                                                                            1E-04 0.04  0.39                                                        85.9364                                                                             0.0003                                                                              0.13  0.52                                                        67.2501                                                                             0.0004                                                                              0.17  0.69                                                        51.7356                                                                             0.0003                                                                              0.13  0.82                                                        40.3645                                                                             0     0.00  0.82                                                        31.4021                                                                             0.0001                                                                              0.04  0.86                                                        24.5747                                                                             0.0002                                                                              0.09  0.95                                                        19.1522                                                                             -0.0001                                                                             -0.04 0.90                                                        14.9049                                                                             0.0001                                                                              0.04  0.95                                                        12.3548                                                                             0.0003                                                                              0.13  1.08                                                        9.0401                                                                              0.0002                                                                              0.09  1.16                                                        7.0369                                                                              -0.0002                                                                             -0.09 1.08                                                        6.0369                                                                              0.0004                                                                              0.17  1.25                                                        4.2819                                                                              0.0004                                                                              0.17  1.42                                                        3.3411                                                                              0.0013                                                                              0.56  1.98         3.3411                                                                              0.0013                                                                              0.56                               2.5936                                                                              0.0017                                                                              0.73  2.71         2.5936                                                                              0.0017                                                                              0.73                               2.0211                                                                              0.003 1.29  4.00         2.0211                                                                              0.003 1.29                               1.5729                                                                              0.0049                                                                              2.11  6.11         1.5729                                                                              0.0049                                                                              2.11                               1.2222                                                                              0.0058                                                                              2.50  8.61         1.2222                                                                              0.0058                                                                              2.50                               0.9503                                                                              0.0068                                                                              2.93  11.54        0.9503                                                                              0.0068                                                                              2.93                               0.7426                                                                              0.0065                                                                              2.80  14.33        0.7426                                                                              0.0065                                                                              2.80                               0.5763                                                                              0.007 3.01  17.35        0.5763                                                                              0.007 3.01                               0.4497                                                                              0.0065                                                                              2.80  20.15        0.4497                                                                              0.0065                                                                              2.80                               0.3491                                                                              0.0058                                                                              2.50  22.64        0.3491                                                                              0.0058                                                                              2.50                               0.2721                                                                              0.0046                                                                              1.98  24.62        0.2721                                                                              0.0046                                                                              1.98                               0.2125                                                                              0.0045                                                                              1.94  26.56        0.2125                                                                              0.0045                                                                              1.94                               0.1629                                                                              0.0059                                                                              2.54  29.10        0.1629                                                                              0.0059                                                                              2.54                               0.128 0.113 4.86  33.96        0.128 0.113 4.86                               0.1004                                                                              0.022 9.47  43.44        0.1004                                                                              0.022 9.47                               0.0782                                                                              0.0283                                                                              12.18 55.62  total       0.0976                                                                              42.01                              0.0628                                                                              0.0262                                                                              11.28 66.90  double check                                                                              42.01 42.01                              0.0606                                                                              0.0006                                                                              0.26  67.15  answer is   42.01                                    0.0474                                                                              0.025 10.76 77.92                                                       0.0369                                                                              0.0162                                                                              6.97  84.89                                                       0.0313                                                                              0.0064                                                                              2.76  87.65                                                       0.0287                                                                              0.003 1.29  88.94                                                       0.0223                                                                              0.0106                                                                              4.56  93.50                                                       0.0174                                                                              0.0039                                                                              1.68  95.18                                                       0.0136                                                                              0.0048                                                                              2.07  97.24                                                       0.0106                                                                              0.0016                                                                              0.69  97.93                                                       0.0082                                                                              0.0017                                                                              0.73  98.67                                                       0.0064                                                                              0.0012                                                                              0.52  99.18                                                       0.005 0.0006                                                                              0.26  99.44                                                       0.0039                                                                              0.0006                                                                              0.26  99.70                                                       0.0031                                                                              0.0007                                                                              0.30  100.00                                                   total    0.2323                                                                              100.00                                                         __________________________________________________________________________

                                      TABLE 10                                    __________________________________________________________________________    Excel DATA 1.128                                                              PORE     Incremental                                                                         Incremental                                                                         Cummulative  PORE  Incremental                                                                         Incremental                     DIAMETER Intrusion                                                                           intrustion                                                                          Intrusion    DIAMETER                                                                            Intrusion                                                                           Intrusion                       μm    mL/g  Per Cent                                                                            Per Cent     μm mL/g  Per Cent                        __________________________________________________________________________       300   0     0.00  0.00                                                        178.9773                                                                            0.0004                                                                              0.17  0.17                                                        144.1443                                                                            0.0006                                                                              0.25  0.42                                                        120.2743                                                                            0     0.00  0.42                                                        85.8054                                                                             0.0005                                                                              0.21  0.63                                                        67.3306                                                                             0.0001                                                                              0.04  0.67                                                        51.7594                                                                             0     0.00  0.67                                                        40.2635                                                                             0.0003                                                                              0.13  0.80                                                        31.3322                                                                             0.0006                                                                              0.25  1.05                                                        24.564                                                                              0.0006                                                                              0.25  1.31                                                        19.1294                                                                             -0.0005                                                                             -0.21 1.10                                                        14.901                                                                              -0.0017                                                                             -0.72 0.38                                                        12.3575                                                                             0.0002                                                                              0.08  0.46                                                        9.0423                                                                              0.0014                                                                              0.59  1.05                                                        7.0172                                                                              0.0021                                                                              0.88  1.94                                                        6.0366                                                                              -0.0007                                                                             -0.29 1.64                                                        4.2818                                                                              0.001 0.42  2.06                                                        3.3315                                                                              0.0019                                                                              0.80  2.87         3.3315                                                                              0.0019                                                                              0.80                               2.5913                                                                              0.0031                                                                              1.31  4.17         2.5913                                                                              0.0031                                                                              1.31                               2.0198                                                                              0.0046                                                                              1.94  6.11         2.0198                                                                              0.0046                                                                              1.94                               1.5714                                                                              0.0063                                                                              2.65  8.77         1.5714                                                                              0.0063                                                                              2.65                               1.2171                                                                              0.0083                                                                              3.50  12.26        1.2171                                                                              0.0083                                                                              3.50                               0.9475                                                                              0.0084                                                                              3.54  15.80        0.9475                                                                              0.0084                                                                              3.54                               0.7399                                                                              0.0077                                                                              3.24  19.05        0.7399                                                                              0.0077                                                                              3.24                               0.5758                                                                              0.0064                                                                              2.70  21.74        0.5758                                                                              0.0064                                                                              2.70                               0.45  0.0048                                                                              2.02  23.77        0.45  0.0048                                                                              2.02                               0.3499                                                                              0.0042                                                                              1.77  25.54        0.3499                                                                              0.0042                                                                              1.77                               0.2726                                                                              0.0036                                                                              1.52  27.05        0.2726                                                                              0.0036                                                                              1.52                               0.2126                                                                              0.0035                                                                              1.47  28.53        0.2126                                                                              0.0035                                                                              1.47                               0.2726                                                                              0.0038                                                                              1.52  27.05        0.2726                                                                              0.0036                                                                              1.52                               0.2126                                                                              0.0035                                                                              1.47  28.53        0.2128                                                                              0.0035                                                                              1.47                               0.163 0.0044                                                                              1.85  30.38        0.0044                                                                              1.85                                     0.1284                                                                              0.007 2.95  33.33        0.1284                                                                              0.007 2.95                               0.1002                                                                              0.016 6.74  40.08        0.1002                                                                              0.016 6.74                               0.0781                                                                              0.0254                                                                              10.70 50.78  total       0.0902                                                                              38.01                              0.0627                                                                              0.0256                                                                              10.79 61.57  double check                                                                              38.01 38.01                              0.0605                                                                              0.0008                                                                              0.34  61.90  answer is   38.01                                    0.0474                                                                              0.0285                                                                              12.01 73.91                                                       0.0369                                                                              0.0187                                                                              7.88  81.80                                                       0.03131                                                                             0.004 1.69  83.48                                                       0.0287                                                                              0.0047                                                                              1.98  85.46                                                       0.0224                                                                              0.0141                                                                              5.94  91.40                                                       0.0174                                                                              0.0062                                                                              2.61  94.02                                                       0.0136                                                                              0.0053                                                                              2.23  96.25                                                       0.0106                                                                              0.0028                                                                              1.18  97.43                                                       0.0082                                                                              0.002 0.84  98.27                                                       0.0064                                                                              0.0013                                                                              0.55  98.82                                                       0.005 0.001 0.42  99.24                                                       0.0039                                                                              0.0008                                                                              0.34  99.58                                                       0.0031                                                                              0.001 0.42  100.00                                                   total    0.2373                                                                              100.00                                                         __________________________________________________________________________

                                      TABLE 11                                    __________________________________________________________________________    Excel DATA 1.127 (Batch 25)                                                   PORE     Incremental                                                                         Incremental                                                                         Cummulative  PORE  Incremental                                                                         Incremental                     DIAMETER Intrusion                                                                           intrustion                                                                          Intrusion    DIAMETER                                                                            Intrusion                                                                           Intrusion                       μm    mL/g  Per Cent                                                                            Per Cent     μm mL/g  Per Cent                        __________________________________________________________________________       300   0     0.00  0.00                                                        178.9773                                                                            0.0003                                                                              0.10  0.10                                                        144.1443                                                                            0.0004                                                                              0.13  0.22                                                        120.2743                                                                            0     0.00  0.22                                                        85.8054                                                                             0.0007                                                                              0.22  0.45                                                        67.3306                                                                             0.0004                                                                              0.13  0.58                                                        51.7594                                                                             0.0006                                                                              0.19  0.77                                                        40.2635                                                                             0.0003                                                                              0.10  0.87                                                        31.3322                                                                             0.0002                                                                              0.06  0.93                                                        24.564                                                                              -1E-04                                                                              -0.03 0.90                                                        19.1294                                                                             0.0004                                                                              0.13  1.03                                                        14.901                                                                              0.0011                                                                              0.35  1.38                                                        12.3575                                                                             0.0004                                                                              0.13  1.51                                                        9.0423                                                                              0.0003                                                                              0.10  1.60                                                        7.0172                                                                              0.0005                                                                              0.16  1.76                                                        6.0366                                                                              0.0015                                                                              0.48  2.24                                                        4.2595                                                                              0     0.00  2.24                                                        3.3178                                                                              0.0016                                                                              0.51  2.76         3.3178                                                                              0.0016                                                                              0.51                               2.5829                                                                              0.0036                                                                              1.15  3.91         2.5829                                                                              0.0036                                                                              1.15                               2.0148                                                                              0.0065                                                                              2.08  5.99         2.0148                                                                              0.0065                                                                              2.08                               1.5685                                                                              0.0086                                                                              2.76  8.75         1.5685                                                                              0.0086                                                                              2.76                               1.2153                                                                              0.011 3.53  12.28        1.2153                                                                              0.011 3.53                               0.9466                                                                              0.0105                                                                              3.37  15.64        0.9466                                                                              0.0105                                                                              3.37                               0.7394                                                                              0.0097                                                                              3.11  18.75        0.7394                                                                              0.0097                                                                              3.11                               0.5755                                                                              0.0086                                                                              2.76  21.51        0.5755                                                                              0.0086                                                                              2.76                               0.4498                                                                              0.0071                                                                              2.28  23.78        0.4498                                                                              0.0071                                                                              2.28                               0.3498                                                                              0.0062                                                                              1.99  25.77        0.3498                                                                              0.0062                                                                              1.99                               0.2725                                                                              0.0057                                                                              1.83  27.60        0.2725                                                                              0.0057                                                                              1.83                               0.2125                                                                              0.0069                                                                              2.21  29.81        0.2125                                                                              0.0069                                                                              2.21                               0.163 0.01  3.21  33.01        0.163 0.01  3.21                               0.1284                                                                              0.015 4.81  37.82        0.1284                                                                              0.015 4.81                               0.1002                                                                              0.0249                                                                              7.98  45.80        0.1002                                                                              0.0249                                                                              7.98                               0.0781                                                                              0.0293                                                                              9.39  55.19  total       0.1359                                                                              43.56                              0.0627                                                                              0.0288                                                                              9.23  64.42  double check                                                                              43.56 43.56                              0.0605                                                                              0.0011                                                                              0.35  64.78  answer is   43.56                                    0.0474                                                                              0.0321                                                                              10.29 75.06                                                       0.0369                                                                              0.02  6.41  81.47                                                       0.0313                                                                              0.0053                                                                              1.70  83.17                                                       0.0287                                                                              0.0005                                                                              1.60  84.78                                                       0.0224                                                                              0.0149                                                                              4.78  89.55                                                       0.0174                                                                              0.007 2.24  91.79                                                       0.0136                                                                              0.0057                                                                              1.83  93.62                                                       0.0106                                                                              0.0042                                                                              1.35  94.97                                                       0.0082                                                                              0.003 0.96  95.93                                                       0.0064                                                                              0.0037                                                                              1.19  97.12                                                       0.005 0.0027                                                                              0.87  97.98                                                       0.0039                                                                              0.0031                                                                              0.99  98.97                                                       0.0031                                                                              0.0032                                                                              1.03  100.00                                                   total    0.312 100.00                                                         __________________________________________________________________________

Each of the catalysts of Examples 2-9 was performance tested in a fixedbed maleic anhydride reactor at a standardized set of reactionconditions--2.4±0.2 mole % n-butane in synthetic air (21 mol % oxygen/71mol % helium, 1.034×10² kPa-g (15.0 psig) inlet pressure, and 1500 GHSV.The catalyst (11.7 g) was charged to a 1.092 cm inside diameter×30.48 cmlong (0.43 inch inside diameter by 1' long) reactor to provide acatalyst bed of approximately 15.24 cm (6") in length. The catalyst wasrun for a period of time from about 20 hours to about 100 hours unlessotherwise indicated at the standardized performance test conditionsprior to determining the reaction (bath) temperature and reaction yield.The reaction (bath) temperature and maximum yield were determined foreach catalyst when the catalyst was running at 85±2 mol % n-butaneconversion. The parameters and results are tabulated in Table 12.

                                      TABLE 12                                    __________________________________________________________________________                                  BCY      AVG. STAND.                                                                           YIELD                                ANST                                                                              TIME BATH                                                                              CONV                                                                              FCY.sup.2                                                                        BCY.sup.1                                                                         AT 85%                                                                             CTOT.sup.3                                                                        BATH                                                                              YIELD                                                                             DELTA                          EXAMPLE                                                                             BATCH                                                                             (HOURS)                                                                            (°C.)                                                                      (%) (%)                                                                              (%) (%)  (%) (°C.)                                                                      (%) (%) VOX                        __________________________________________________________________________    2     6   108  411 84.8                                                                              59.1                                                                             60.1                                                                              60.2 98.0                                                                              421 56.6                                                                              3.7 4.21                       2     6   138  411 85.3                                                                              59.7                                                                             59.5                                                                              59.3 99.2                                                                              421 56.6                                                                              2.7 4.21                       3     7    87  423 85.0                                                                              58.5                                                                             58.1                                                                              58.1 99.3                                                                              421 56.6                                                                              1.5 4.14                       3     7   124  419 84.9                                                                              56.4                                                                             57.6                                                                              57.7 97.8                                                                              421 56.6                                                                              1.1 4.14                       3     7   101  424 85.2                                                                              56.3                                                                             57.0                                                                              56.9 98.3                                                                              421 56.6                                                                              0.3 4.14                       4     9    84  430 85.0                                                                              56.5                                                                             56.2                                                                              56.2 99.2                                                                              421 56.6                                                                              -0.4                                                                              4.19                       4     9    83  413 85.3                                                                              58.8                                                                             58.7                                                                              58.5 99.2                                                                              421 56.6                                                                              1.9 4.19                       4     9   102  423 85.5                                                                              57.2                                                                             57.8                                                                              57.5 98.4                                                                              421 56.6                                                                              0.9 4.19                       5     21   92  411 85.5                                                                              56.9                                                                             61.4                                                                              61.0 94.5                                                                              420 57.5                                                                              3.5 4.16                       6     22  101  411 85.2                                                                              58.0                                                                             59.4                                                                              59.3 97.6                                                                              420 57.5                                                                              1.7 4.16                       7     23  108  411 85.2                                                                              59.7                                                                             61.1                                                                              61.0 97.5                                                                              420 5.5 3.4 4.18                       8     24   97  418 85.5                                                                              58.5                                                                             59.6                                                                              59.3 97.9                                                                              420 57.5                                                                              1.7 4.18                       9     25   95  413 85.5                                                                              59.8                                                                             60.1                                                                              59.7 98.7                                                                              420 57.5                                                                              2.2 4.17                       Average        416.8                                                                             85.2                                                                              58.1                                                                             59.0                                                                              58.8 98.1                                                                              420.6                                                                             56.9                                                                              1.88                                                                              4.17                       Standard Deviation                                                                           6.24                                                                              0.23                                                                              1.26                                                                             1.51                                                                              1.45 1.22                                                                              0.88                                                                              0.47                                                                              1.19                                                                              0.02                       __________________________________________________________________________     .sup.1 % BCY (% Back Calculated Yield) and Bath Activity Temperature          (°C. required to give 85 +/- 1% butane concentration) were obtaine     from microreactors running at 1500 GHSV, 15 psig, and 2.4 mole % butane.      .sup.2 (Moles maleic produced/moles nbutane feed) (100%)                      .sup.3 Closure of carbon balance                                         

EXAMPLE 10

Differential scanning calorimetry tests were conducted after conclusionof the pore builder removal step on catalyst precursor bodies preparedin the essentially the same manner as the precursor bodies of Examples1-9. For purposes of comparison, additional differential scanningcalorimetry tests were conducted on catalyst precursor bodies preparedin the essentially the same manner except that the pore builder removalstep was terminated after at least 16 hours at 165° C.; and furthertests were conducted on a catalyst of the prior art which had beenproduced without the use of any pore builder. The results of these testsare illustrated in the DSC readout plot of FIG. 6.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above compositions and processeswithout departing from the scope of the invention, it is intended thatall matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A process for the preparation of a phosphorus/vanadium oxide catalyst comprising the steps of:preparing a modified catalyst precursor composition comprising a mixture of a particulate phosphorus/vanadium oxide catalyst precursor and a volatile pore modification agent in proportions sufficient to provide a pore modification agent concentration of between about 8% and about 16% by weight, said pore modification agent having a vapor pressure of at least about 1 mm Hg at a temperature between about 150° C. and about 250° C. and being subject to vaporization, decomposition and/or oxidation at a temperature below 300° C. without leaving a substantial residue; forming said modified catalyst precursor composition into a predetermined shape under compression, thereby producing a shaped porous catalyst precursor body comprising said catalyst precursor composition and containing said volatile pore modification agent; and heating said precursor body while passing a stripping gas consisting essentially of air thereover for removal of at least about 80% by weight of said volatile pore modification agent at a temperature not greater than about 300° C., said porous body being heated at a rate of between about 1° and about 3° C. per minute to a hold temperature that is below a threshold temperature at which the pore modification agent is subject to catalytic oxidation in the presence of the phosphorus/vanadium oxide catalyst precursor.
 2. A process as set forth in claim 1 wherein the stripping gas flows over the precursor body at a linear rate of at least about 25 cm/sec.
 3. A process as set forth in claim 2 wherein said pore modification agent substantially comprises stearic acid, and said hold temperature is between about 160° C. and about 170° C.
 4. A process as set forth in claim 2 wherein the shaped body is heated from said hold temperature to a terminal temperature at which the concentration of residual pore modification agent in the shaped body has been reduced to a terminal level, the terminal level being defined as a concentration of residual pore modification agent which does not result in an exotherm that adversely alters the catalyst structure upon subsequent transformation of the precursor composition to active catalyst by heat treatment of the shaped body in the presence of air, steam or nitrogen under reference conditions, such reference conditions comprising heating said precursor at a rate of 1.8° C. per minute to between 300° C. and 400° C.
 5. A process as set forth in claim 1 wherein said shaped porous body is heated to a temperature between about 150° C. and about 250° C. for removal of said pore modification agent.
 6. A process as set forth in claim 1 wherein said pore modification agent substantially comprises stearic acid, and said hold temperature is between about 160° C. and about 170° C.
 7. A process as set forth in claim 1 wherein said pore modification agent is removed by vaporization.
 8. A process as set forth in claim 1 wherein essentially complete removal of said agent is provided, without reduction of vanadium in said precursor composition to an average oxidation state of less than about 3.8.
 9. A process as set forth in claim 1 wherein said pore modification agent is selected from the group consisting of fatty acids, fatty acid esters, and polynuclear organic compounds.
 10. A process as set forth in claim 1 wherein said particulate phosphorus/vanadium oxide precursor is mixed with a particulate pore modification agent, the mean particle diameter of said pore modification agent being not greater than about two orders of magnitude different from the mean particle diameter of said precursor.
 11. A process as set forth in claim 1 wherein, after removal of said pore modification agent, said precursor body is heated at a temperature above 300° C. to transform said precursor body to a phosphorus/vanadium oxide catalyst body active for the oxidation of a hydrocarbon to maleic anhydride. 